Adds cinder.keymgr.fixed_key to cinder element

Not having keymgr.fixed_key causes tons of WARNING messages like:

 WARNING cinder.keymgr.conf_key_mgr [-] config option keymgr.fixed_key
 has not been defined: some operations may fail unexpectedly

Once this lands we can add a bit of overcloud metadata so we set
a reasonable default for this (perhaps 'unset' even)...

Change-Id: I8cf41379aae0e0ed460c4b27f7279796c8ad8f8d
Merge "Adds cinder.keymgr.fixed_key to cinder element"