neutron: added test case to check connectivity using MTU sized frames

Properly configured setup must allow frames of network MTU size (as
reported by Neutron net-mtu API extension) through the network.

Sadly, Cirros images ship ping tools that don't support the -M argument
that allows to set Don't Fragment flag, so the only way to enforce it is
from outside the instance.

Change-Id: I1dd0633e48dbc9b67cd79e610e959b514304a4b5
Related-Bug: #1623876
4 files changed