Includes:
* Remove leader upgrade raft version test, as older versions of raft are now
incompatible with our autopilot library.
* Remove attempt to assert initial non-voter status on the `PromoteNonVoter`
test, as this happens too quickly to reliably detect.
* Unskip some previously-skipped tests which we should make stable.
* Remove the `consul/sdk` retry helper for these tests; this uses panic recovery
in a kind of a clever/gross way to reduce LoC but it seems to introduce some
timing issues in the process.
* Add more test step logging and reduce logging noise from the scheduler
goroutines to make it easier to debug failing tests.
* Be more consistent about using the `waitForStableLeadership` helper so that we
can assert the cluster is fully stable and not just that we've added peers.
Nomad's original autopilot was importing from a private package in Consul. It
has been moved out to a shared library. Switch Nomad to use this library so that
we can eliminate the import of Consul, which is necessary to build Nomad ENT
with the current version of the Consul SDK. This also will let us pick up
autopilot improvements shared with Consul more easily.
