If apt_repository fails to update the apt cache after updating the
configuration, retries don't register there was a change and so no
attempt is made to update the cache by the module on the second attempt.
This failure can result in a failure to install packages.
This change adds an apt module task to update the cache if the
apt_repository task registers a change. This means updating the cache
will get retried on failure and no longer fail silently.
Backport includes: https://review.openstack.org/547471
Change-Id: Id059dbec3466cb1ef3ea567249f52384a8ade515
Partial-Bug: #1750656
(cherry picked from commit 23bd341129)
cffd1ebd45