neutron: remove tenant MTU configuration options

neutron: remove tenant MTU configuration options

Since Mitaka, Neutron and Nova do the right thing for MTU, correctly
calculating and applying MTU per network, considering its network type
and underlying physical network MTU (1500 by default). Neutron now also
correctly advertise proper MTU to instances through DHCP and RA
mechanisms. With that, there is no reason to have those MTU hacks in
deployment tools. Actually, they not only do no good, but break some
setups (Jumbo frame aware infrastructure), or at least make them
non-optimal (lowering instance MTU to 1400 when it's not needed, or when
tunnel overhead does not require 100 bytes).

Note that Neutron still has a set of configuration options to allow for
custom physical network MTUs (global_physnet_mtu, path_mtu,
physical_network_mtus). Those options define underlying infrastructure
though, not tenant MTUs. To support Jumbo frames, TripleO should allow
to set those options. That said, it's not the immediate goal of the
patch, and hence such an effort would require a separate patch.

Mitaka+ documentation on MTU configuration for Neutron:
http://docs.openstack.org/mitaka/networking-guide/adv-config-mtu.html

Change-Id: I540ba5dc69d0506f71b59746efcce94c73f9317f