Target ec2-user workaround to Ubuntu 12.10.

Ubuntu 13.04 has been confirmed to work properly, so only Ubuntu 12.10
is affected by the bug (http://pad.lv/1100920). Thus, we can narrow the
scope of the workaround to just "Ubuntu quantal".

Change-Id: Ib42e2bce2f4d0d2cfd6bdeb70cfa4162c2cd6433
Merge "Target ec2-user workaround to Ubuntu 12.10."