Adjust MTU to compensate for VLAN tag issue
Adjust MTU to compensate for VLAN tag issue

Some network configurations uncovered what appears to be an issue where
a spurious 802.1q header is injected into tunnelled traffic. Adjusting
the default value to accomodate the extra overhead should avoid this
problem.

Partial-Bug: #1621533
Change-Id: I9ebad2d6ad34d90fcb998497873059995cdef276

dprince/tripleo-heat-templates,openstack/tripleo-heat-templates,trozet/opnfv-tht,dprince/tripleo-heat-templates,trozet/opnfv-tht,trozet/opnfv-tht,openstack/tripleo-heat-templates