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

setup.py

@@ -66,14 +66,7 @@ using_GPU = out != ''
 
 if not using_GPU:
     if not slim_package:
-        # For linux wth no GPU, we install the Intel optimized version of TensorFlow
-        if sys.platform == "linux" or sys.platform == "linux2":
-            # CI: limiting version to 1.13.1 due to
-            # https://github.com/tensorflow/tensorflow/issues/29617
-            # (reproduced with intel-tensorflow 1.14.0 but not with 1.13.1)
-            install_requires.append('intel-tensorflow==1.13.1')
-        else:
-            install_requires.append('tensorflow>=1.9.0,<=1.14.0')
+        install_requires.append('tensorflow>=1.9.0,<=1.14.0')
     extras['mxnet'] = ['mxnet-mkl>=1.3.0']
 else:
     if not slim_package:
@@ -94,7 +87,7 @@ setup(
     author='Intel AI Lab',
     author_email='coach@intel.com',
     packages=find_packages(),
-    python_requires=">=3.5.*",
+    python_requires=">=3.6.*",
     install_requires=install_requires,
     extras_require=extras,
     package_data={'rl_coach': ['dashboard_components/*.css',