This commit delivers an isolated component of AIP-61:
https://cwiki.apache.org/confluence/display/AIRFLOW/AIP-61+Hybrid+Execution

It updates the executor configuration logic to allow configuring multiple
executors (and aliases per executor) in a list form (comma delimited), as
well as associated methods for core Airflow code to load the default executor
(first executor in the list) or any other configured executor.

Testing is included with the changes but not documentation, because the
feature is not yet complete and is currently disabled. User facing
documentation will be delivered when the entire AIP-61 feature is
released.

Executor data is now more aggressively cached and this was failing some tests
that are trying to override the executor conf. Moved the caches to the module
level and added some reload(executor_loader) calls to empty the caches for
certain tests that require it.

This unit test was broken to begin with and was working mostly by
chance. Updated the patch to be the correct method to mock and test
is passing again