Permit specifying VLAN mappings to overclouds.

To use a VLAN based public network we need the ext-net network to be a
VLAN with a segmentation id - but we can't do this unless we also have
the datacentre physical network marked as allowing vlans.

We could make this strictly opt-in, but as this doesn't affect the
switch configuration (and thus actual machine capabilities) having it
on by default seems reasonable. OTOH we can't force it on, because
high security environments may well want a defense in depth setup
where neutron admins cannot configure VLANs that they are not meant
to have access too (consider that the cloud machine admins may be
separate to the folk running the services on top of them...)

Change-Id: I9687751753f810896c6d065750910da40132c9fa
Merge "Permit specifying VLAN mappings to overclouds."