Add distributed flag for neutron-fwaas traceback

New flag was introduced to neutron-fwaas create firewall for DVR. cfg.CONF.router_distributed flag is added to prevent traceback.

 def create_firewall(self, context, firewall):
LOG.debug("create_firewall() called")
fw = firewall['firewall']
tenant_id = self._get_tenant_id_for_create(context, fw)
# distributed routers may required a more complex state machine;
# the introduction of a new 'CREATED' state allows this, whilst
# keeping a backward compatible behavior of the logical resource.
status = (const.CREATED
if cfg.CONF.router_distributed else const.PENDING_CREATE)