Update neutron network node section to adjust MTU

I updated the neutron network node section to discuss the
implications of tunneling protocols such as GRE, provide
potential solutions, and recommend using DHCP to adjust the
instance MTU.

This patch does not address the equivalent section for the
deprecated OVS plug-in.

Change-Id: Ia8dd9fe78d5c845c85bfde0d96481a55f754e121
Closes-Bug: #1322799
Merge "Update neutron network node section to adjust MTU"