Use oslo.test mockpatch

Keystone was using mockpatch from oslo-incubator. Since oslo has
migrated these parts to oslo.test, switch to the oslo.test versions.

Note that these modules aren't mentioned in openstack-common.conf,
otherwise this change would remove it.

Change-Id: I2d4cd0b3e980d397b0111f25b2f645ae09dd9199
Merge "Use oslo.test mockpatch"