Use moxstubout and mockpatch from oslotest

Modules moxstubout and mockpatch from openstack/common/fixture/
are obsolete, so we should use them from oslotest library.

Also oslotest depends on mock and mox, so we no longer need
to specify them directly in test-requirements files

Change-Id: I139133457cd73ee83537dead3446ee30d0ee5589
Merge "Use moxstubout and mockpatch from oslotest"