Note that install guide also works for 12.04

I had feedback to the effect that people were ignoring this
install guide because it appeared not to work for ubuntu 12.04,
which is untrue.

Explictly labelling this means that those looking to run openstack
on Ubuntu's Long Term Supported release will know this guide can
also be used.

Change-Id: I61940132ac2bc26c91dde95713e48123db45fb15
Merge "Note that install guide also works for 12.04"