Use unittest.mock instead of mock

The mock third party library was needed for mock support in py2
runtimes. Since we now only support py36 and later, we can use the
standard lib unittest.mock module instead.

Change-Id: Id3007be0973e7da345aa9b02c6abde25a373180f

test-requirements.txt

@@ -6,7 +6,6 @@ hacking>=0.12.0,!=0.13.0,<0.14  # Apache-2.0
 coverage>=3.6
 discover
 fixtures>=1.3.1
-mock>=1.2
 python-subunit>=0.0.18
 sphinx!=1.2.0,!=1.3b1,<1.3,>=1.1.2
 oslosphinx>=2.5.0 # Apache-2.0