Support automatic rollback model

After an update has completed a system should perform self test (monitor for boot timeout, run sanity checks on the new default system once it has booted) and automatically rollback if necessary.

REF:
https://mail.gnome.org/archives/ostree-list/2015-July/msg00006.html
https://bugzilla.gnome.org/show_bug.cgi?id=725346
https://github.com/projectatomic/rpm-ostree/issues/177
http://www.denx.de/wiki/view/DULG/UBootBootCountLimit
https://fedoraproject.org/wiki/Summer_coding_ideas_for_2016#Automatic_Atomic_Host_Updates

Some inspiration : https://github.com/endlessm/eos-updater/pull/39/commits/4bc76d920c3591f27590bbefd5038e7a16405820 and https://github.com/endlessm/eos-boot-helper/pull/56