Add doc note about over-mtu packet loss.
Add doc note about over-mtu packet loss.

In some cases (on Fedora 19, using *real*
bare metal, etc) users have seen over-mtu packet
loss when running iSCSI commands. In these
cases:

  iscsid: log shows repeated connection failed errors (and reconnect)

  dmesg shows:
    openvswitch: vnet1: dropped over-mtu packet: 1502 > 1500

This patch makes a note in devtest so that people who encounter
similar errors may know they need to increase their MTU
for the control plane.

Change-Id: Iefdb0ab31769bc0a9cd15eab3948f3d78c35efdd

rdo-management/tripleo,rdo-management/tripleo