Remove tripleo PPA from base element.

The PPA would have to be updated for each release we want to build for,
including raring. The PPA also gives the tripleo team root on every
built image, something we don't necessarily want.

A shallow review of the packages in the PPA and known elements shows
only one dependency in the tripleo-image-elements heat-cfntools element.

Change-Id: Ic514f929eba26014dd3a0d76d5bb3b8e9e0caf3c
Merge "Remove tripleo PPA from base element."