Implement help for v2 bridge

Implement help message generation for the noun/verb framework.

Bugs closed: aurora-203

Reviewed at https://reviews.apache.org/r/18267/
AURORA-186: rename ThermosExecutor to AuroraExecutor, clean up thermos-isms from ExecutorBase

split all thermos-specific stuff out of thermos_executor.py

Testing Done:

this adds executor-small and executor-large, the latter of which runs the
executor tests.

Both ./pants src/test/python/apache/aurora/executor:executor-large and e2e
passes locally for me.

Bugs closed: AURORA-186

Reviewed at https://reviews.apache.org/r/18239/
Adding admin_client sla_list_safe_domain command.

Bugs closed: AURORA-208

Reviewed at https://reviews.apache.org/r/18296/
Renamed getJobSummary call to getRoleSummary

getJobSummary will be reused to return the jobs.

Testing Done:
gradle clean build
gradle run and tested on my laptop

Bugs closed: AURORA-64

Reviewed at https://reviews.apache.org/r/18340/
Prepare and launch GC executor tasks asynchronously.

Bugs closed: AURORA-214

Reviewed at https://reviews.apache.org/r/18141/
Updated FakeSchedulerDriver to work with delayed driver registration semantics of SchedulerLifeCycle.

Updated FakeSchedulerDriver to register scheduler on start instead of run.

Testing Done:
gradle clean build.
rm /tmp/testing_log_file && gradle run

Bugs closed: AURORA-179

Reviewed at https://reviews.apache.org/r/18300/
AURORA-170: expose process exit status via observer UI