Pre-cache UCA packages during nodepool img build

While caching packages required by devstack, temporarily enable
the Ubuntu Cloud Archive for specified pockets and do additional
deb package caching.  This will pre-cache UCA versions of devstack
dependencies alongside the standard 12.04 requirements. After they
have been cached, the repository access is removed.  This allows
standard devstack jobs to use the vanilla 12.04 packages as usual,
and jobs that require UCA can enable it and use it without hitting
the network for either the GPG key or the packages.

Change-Id: I1cad663a71a3129dc97347dd4d39a760e432aff2
Closes-bug: #1329430.
Merge "Pre-cache UCA packages during nodepool img build"