Merge lp:~fginther/cloudware/ubuntu-repository-cache-add-tests into lp:~cloudware/cloudware/ubuntu-repository-cache

Add 'mirror-series' option to reduce the amount of metadata synced and speed up functional tests. Add functional tests for cron syncing, testing rsync retries and unit fail-over.

[a=fginther] [r=tribaal]
MP: https://code.launchpad.net/~fginther/cloudware/ubuntu-repository-cache-add-tests/+merge/321850