Adds openvswitch-datapath-dkms package install for Ubuntu

Adds a note with troubleshooting information when GRE doesn't work
on Ubuntu 12.04 LTS.

Change-Id: I6223bbc0a0e77b1b0971ed52d7bbc7266d5fc69b
Closes-bug: 1249589
Partial-bug: 1238445
(cherry picked from commit fd01182026551015aa886d24ec5a9d68fde23b55)