Introduce the neutron-wide 'global_physnet_mtu' option that
references the underlying physical network MTU. This also
introduces a method in plugin.common.utils that all plugins
should use to retrieve it. This value should be used to
calculate the proper MTU for virtual network components.
This patch also deprecate the 'segment_mtu' option specific
to the ML2 plug-in and makes ML2 reference this new option.
Closes-Bug: #1542475
Closes-Bug: #1542108
Change-Id: I6ffc8973c9b8f46cc19922ff04fdd2d23646b878