Warn about losing connectivity with add-port

Add a warning about how "ovs-vsctl add-port" will cause
loss of network connectivity if there is an IP associated with
the corresponding interface.

This issue came up on the openstack-operators mailing list:
http://lists.openstack.org/pipermail/openstack-operators/2014-January/003831.html

Change-Id: I1c5d266cc3fd728cfe8638ced78ec1cf4d102c3c
Merge "Warn about losing connectivity with add-port"