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/