Now that we are python3 only, we should move to using the built
in version of mock that supports all of our testing needs and
remove the dependency on the "mock" package.
This patch moves all references to "import mock" to
"from unittest import mock". It also cleans up some new line
Fixed an inconsistency in the OVSBridge.deferred() definition
as it needs to also have an *args argument.
Fixed an issue where an l3-agent test was mocking
functools.partial, causing a python3.8 failure.
Unit tests only, removing from tests/base.py affects
functional tests which need additional work.