Lazy log the fixed_key warnings

Cinder currently spews quite a bit of WARNINGs like this with
the default settings:

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

Many users may not be using features that require key manager...
so logging the WARNING message a bit more lazily seems reasonable.

Change-Id: I5ab72285c7d4bc2ec3196dd94fabf977b3a4ebaf
Closes-bug: 1252512
Merge "Lazy log the fixed_key warnings"