SDNs, overlays, etc often won't need to map ports.

Also don't try to return a DriverNetwork with host networking. None will
ever exist as that's the point of host networking: rkt won't create a
network namespace.

Fixes #3561

The rkt port mapping test currently starts redis with --version, which
obviously makes redis exit again almost immediately. This means that the
container exists before the network status can be queried, and so the
test fails.

To test that the rkt driver correctly sets a DriverNetwork, at least
when a port mapping is requested, we amend the
TestRktDriver_PortsMapping test with a small check.

* alloc_runner

* Random tests

* parallel task_runner and no exec compatible check

* Parallel client

* Fail fast and use random ports

* Fix docker port mapping

* Make concurrent pull less timing dependant

* up parallel

* Fixes

* don't build chroots in parallel on travis

* Reduce parallelism on travis with lxc/rkt

* make java test app not run forever

* drop parallelism a little

* use docker ports that are out of the os's ephemeral port range

* Limit even more on travis

* rkt deadline

Fixes #2478 #2474 #1995 #2294

The new client only handles agent and task service advertisement. Server
discovery is mostly unchanged.

The Nomad client agent now handles all Consul operations instead of the
executor handling task related operations. When upgrading from an
earlier version of Nomad existing executors will be told to deregister
from Consul so that the Nomad agent can re-register the task's services
and checks.

Drivers - other than qemu - now support an Exec method for executing
abritrary commands in a task's environment. This is used to implement
script checks.

Interfaces are used extensively to avoid interacting with Consul in
tests that don't assert any Consul related behavior.

Fixes #2358

This PR fixes our vet script and fixes all the missed vet changes.

It also fixes pointers being printed in `nomad stop <job>` and `nomad
node-status <node>`.

Cleanup can be used for cleaning up resources created by drivers to run
a task. Initially the Docker driver is the only user (to remove
downloaded images).

* Refactor AllocDir to have a TaskDir struct per task.
* Drivers expose filesystem isolation preference
* Fix lxc mounting of `secrets/`

The Driver.Prestart method currently does very little but lays the
foundation for where lifecycle plugins can interleave execution _after_
task environment setup but _before_ the task starts.

Currently Prestart does two things:

* Any driver specific task environment building
* Download Docker images

This change also attaches a TaskEvent emitter to Drivers, so they can
emit events during task initialization.

Signed-off-by: Miquel Sabaté Solà <msabate@suse.com>