Fix incompatibility with the grizzly nova schedular

DHC-1018

* Fix problem due to CONF.node_availability_zone renaming to
  CONF.default_availability_zone
* Fix change in nova API that was causing our overridden
  get_guest_config method to accept a wrong number of argument

Change-Id: I241651f40b3d0381da9381d94d06d7051c7d0fad
Signed-off-by: Rosario Di Somma <rosario.disomma@dreamhost.com>