Drop conf_key_mgr warning message!

Drop conf_key_mgr warning message!

By default ConfKeyManager logs tons of WARNING message stating
that it isn't production ready...

Given that it is currently the only Cinder key manager option
which can be used/selected I don't think repeatedly logging
warnings is helpful. Lets just drop the warning message
for now and when a good "production ready" cinder key manager
implementation is implemented perhaps we can re-add a warning to
this class (hopefully making the production ready impl the default).

Change-Id: Id1fdddc20a963f9fa4749ad57f355cd83d0e14e3
Closes-Bug: #1236459