devstack: switch default odl version to beryllium snapshot
devstack: switch default odl version to beryllium snapshot

As beryllium is released, let's use beryllium as default.

Change-Id: I93e20df9609ff866076343e85cec74ffaa959317

FedericoRessi/networking-odl,openstack/networking-odl,openstack/networking-odl,openstack/networking-odl,FedericoRessi/networking-odl