Clear out ml2_vlan_allocations if CVX has no VLAN pool configured
Clear out ml2_vlan_allocations if CVX has no VLAN pool configured

If there is no vlan pool configured for a region, but CVX was
reachable, the arista_vlan type driver would fail to start.
We should treat this case as there being no VLANs assigned
to the region and clear out all vlan allocations from the db.

Change-Id: Ia57dcd1354ccf98647d7683af8004aebde305ff2

openstack/networking-arista,stackforge/networking-arista,openstack/networking-arista