Reintroduce support for provider networks

There are two problems with provider networks:

First, the way bridge mappings are done
prevents users from having mutliple
provider networks. This patch allows
the user to pass them in directly.

Second, change 38426 proposes changing
the behavior of network_vlan_ranges
and this patch will reflect that by
passing down undef unless the user wishes
to override the defaults specified
in quantum::plugins::ovs

Change-Id: I36b11f9e0fa5458e50cbe59c8f4a5d1f3d9bdaa6
Merge "Reintroduce support for provider networks"