Update to work with Grizzly version of nova after that it moved to oslo.config

DHC-979

As consequence of the transition of nova to oslo.config any
reference to flags.py need to be changed to use the oslo.config
code

Change-Id: Ibab171b6d60c3447ad274c7ef7b7a8e2ef24dd0a
Signed-off-by: Rosario Di Somma <rosario.disomma@dreamhost.com>