Remove duplicated method in mock_key_mgr

The method _generate_key_id was duplicated in
nova/keymgr/mock_key_mgr.py

This patchset removes one of the duplicates.

Change-Id: Idb30fca9d392fe5f5b4063ba1b22a967329987f6
Merge "Remove duplicated method in mock_key_mgr"