Check that shared networks are VLAN networks before in Arista ML2
Check that shared networks are VLAN networks before in Arista ML2

When we added support for shared networks, we introduced a bug that
can result in the arista ML2 driver reacting to a port-create for a
non-vlan network if the network is a shared network. With this
change the arista ML2 driver will only react to port creates for
shared networks if they are vlan networks.

Change-Id: I4babcb82bd3efd7d189f5a284916507be6f41a70
Closes-Bug: #1553058

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