Require Python 3.6 + Changes to CI configuration (#452)

* Change build_*_env jobs to pull base image of current "tag"
  instead of "master" image
* Change nightly flow so build_*_env jobs now gated by build_base (so
  change in previous bullet works in nightly)
* Bugfix in CheckpointDataStore: Call to object.__init__ with
  parameters
* Disabling unstable Doom A3C and ACER golden tests

rl_coach/data_stores/checkpoint_data_store.py

@@ -25,7 +25,6 @@ class CheckpointDataStore(object):
     A DataStore which relies on the GraphManager check pointing methods to communicate policies.
     """
     def __init__(self, *args, **kwargs):
-        super().__init__(*args, **kwargs)
         self.checkpoint_num = 0
 
     def end_of_policies(self) -> bool: