Merge lp://staging/~bjornt/charms/trusty/ceph-radosgw/apt-cache-memonly into lp://staging/~openstack-charmers-archive/charms/trusty/ceph-radosgw/next

Build the Apt cache in memory when checking the ceph version.

This avoids a race condition where something else is rebuilding
the Apt cache in the same directory.

I also changed the imports, so it's clear that it's the apt_pkg
that is being used. There's another module called apt, which
provides a different API, and importing apt_pkg as apt has
fooled me more than once.