Enable changing of rpc driver from devstack
Enable changing of rpc driver from devstack

It enables user to set flag
'MISTRAL_USE_KOMBU_DRIVER' in local.conf on their
devstack. With this flag set to 'TRUE', mistral
would autamiatically be installed with kombu driver
enabled.

This change is needed for adding new devstack gate
that would test kombu driver

Change-Id: I462aa17107a9a55396a87a8c46fa3a8dac338fac

openstack/mistral,StackStorm/mistral,openstack/mistral,StackStorm/mistral