AppVeyor for Windows Server 2016 CI environment

Track task start/finish time & improve logs errors

This PR adds tracking to when a task starts and finishes and the logs
API takes advantage of this and returns better errors when asking for
logs that do not exist.

Fix variable capture and add tests

This PR fixes token revocation and adds tests to make sure it is
working. The 0.5.6 RC1's token revocation does not work becasue the
token's value is captured at the instantiation of the deferred
stoprenewal statement rather than its exectution.

Fix typo and be consistent in naming

Vendor consul & consul-template

Improve artifact download error message

Don't take a reference a var in a loop

disk is not a supported entry at task level

Removed the reference to `disk` in the `resources` overview statement

Stop Vault token renew on task exit

Fix dispatch of periodic job