Add MatchesDictExceptForKeys custom matcher

Add a custom matcher that compares two dictionaries, except for
keys matching an exclusion list.
The matcher returns with a mismatch object design to provide a
useful message in case of failed assertion, which pinpoints the
differences between the two dicts.

Verified via unit tests.

Partially implements: bp/tempest-client-scenarios

Change-Id: Ibdd34fa1f095ee2ee33e11a8e23e654fb6d12b89
Merge "Add MatchesDictExceptForKeys custom matcher"