Move required keymgr classes out of nova/tests

Currently the Nova default keymgr implementation tries to
import classes from nova/tests. This could be a very bad
thing for production deployments which may not include
code from nova/tests.

This change moves two required modules (single_key_mgr and
mock_key_mgr) into the nova/keymgr tree.

Fixes bug: #1224526

Change-Id: I683b0245ab6b6acf8a4ba26f96d8c505f7c7cac8
Merge "Move required keymgr classes out of nova/tests"