Remove custom test assertions.
Remove custom test assertions.

Now that we testtools, We can use stock testtools assert methods
instead of defining our own. As part of removing them, we add Matcher
classes to use in the testtools.assertThat method. testtools matchers can
be arbitrarily combined and chained, so give more flexibility than plain
asserts

Related to blueprint grizzly-testtools.

Change-Id: I26d1dbac8dc3c322eb55c96c48330c0e38636107

openstack/oslo.versionedobjects,citrix-openstack-build/oslo.versionedobjects