reorganize placeholder data call to live with provisioner implementation; force users to use the generated function, therefore forcing validation, for all variables except winrmpassword, by doing a simple string check against the placeholder data.

* Move setting of default EnvVarFormat to the Prepare function
* Add generated hcl2 structure for the updated Shell provisioner config
* Move `EnvVarFormat` field to shared common shell type
* Add description for the `env_var_format` parameter
* Update `env_var_format` when used with `use_env_var_file`

It is simply the best/simplest solution and trying to prevent users from passing and integer here would be like opening a can of worms. Because:

* we cannot make mapstructure validate our duration string ( with an UnmarshalJSON func etc.)
* we cannot make mapstructure spit a string instead of a duration and packer will decode-encode-decode config.
* the hcl2 generated code asks for a string, so this will be enforced by default.

Before this commit it was possible to set a duration using an integer or a float. Go's time.Duration is an int64 internally an mapstructure will take advantage of this and load the number as a int64 but `1` means one ns which is unexpected/confusing. To avoid confusion and enforce readability this forces users to pass a string with a unit for a duration; ex "56s".

for history

* make possible execution policy values lowercase
* prefix execution policies possibilities

this has the effect to not wrap the command, which will allow to know the exit status of a command in docker

* I had to contextualise Communicator.Start and RemoteCmd.StartWithUi
NOTE: Communicator.Start starts a RemoteCmd but RemoteCmd.StartWithUi will run the cmd and wait for a return, so I renamed StartWithUi to RunWithUi so that the intent is clearer.
Ideally in the future RunWithUi will be named back to StartWithUi and the exit status or wait funcs of the command will allow to wait for a return. If you do so please read carrefully https://golang.org/pkg/os/exec/#Cmd.Stdout to avoid a deadlock
* cmd.ExitStatus to cmd.ExitStatus() is now blocking to avoid race conditions
* also had to simplify StartWithUi

this regroups the calls of tmp func calls into one package.
the tmp pkg allows to store tmp files under a common directory for easier managment

Squashed commit of the following:
commit 74b674c01560c6b547e9cab5e8afb486f0fe7d6c
Merge: fc94b5714 66001525d
Author: Matthew Patton <mpatton@enquizit.com>
Date:   Fri Nov 2 02:02:38 2018 -0400

    Merge branch 'remove_TMPDIR_docs' of github.com:tb3088/packer into remove_TMPDIR_docs

commit fc94b571428447aaf2e5171967c95cdf9e5c73ff
Author: Matthew Patton <mpatton@enquizit.com>
Date:   Fri Nov 2 01:42:38 2018 -0400

    update tests to honor configured temporary directory

commit 5a562359f4937af506607dd96ed1c46426093f52
Author: Matthew Patton <mpatton@enquizit.com>
Date:   Thu Nov 1 18:52:38 2018 -0400

    sync straggler to refactor. implement tests at packer/configfile instead of buried in docker.

commit 9d5a4a0935bfd049e2f8f50ad73e4455b42c9410
Author: Matthew Patton <mpatton@enquizit.com>
Date:   Thu Nov 1 15:39:46 2...

Use the common elevated code instead.

Fixes #6104

explicitly use set-variable to work around a bug some users saw where the command invocation would vail

Going to revert this change for now, becuase of potential issues that arise from calling Prepare() twice
Revert "use statebag instead of SetSharedState for winRM password"

This reverts commit b35acbd8.

filter winrm password from logs
Add new root-level packer template option, sensitive-variables, to tell us what user variables to mark sensitive.

* Dollars are interpreted by *nix shells so paths using env vars such as
  ${env:SYSTEMROOT} will cause issues

Previously the location the file was uploaded to was set internally and
used ${env:SYSTEMROOT}/Temp as the destination folder.
By default, in order to inject the required environment variables, the file
is 'dot sourced' by the 'execute_command' using the {{ .Vars }} variable.
Unfortunately the inclusion of the dollar in the path caused issues for
users connecting over ssh as the (typically bash) shell running the execute
command would try and interpret the dollar sign.
The change allows users to specify the location the file is uploaded to,
thereby allowing the user to specify a custom 'execute_command' that
escapes any dollar signs that could be present in the path.
If not set the upload path now defaults to using C:/Windows/Temp as the
upload folder.

use build name to ensure that winrm password and other shared state is not overwritten if two builders need the password in the same packer run.