Assure zero downtime minor-version overcloud updates

EDIT:
For now we will use launchpad bugs to track the lifecycle related work for newton (major upgrades, minor updates and mixed version support). The tag is: https://bugs.launchpad.net/tripleo/+bugs?field.tag=upgrade-bugs

Original bp text below. I will also retarget this bp from n3 now.
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We have to assure ability to perform zero downtime minor-version updates on
overcloud in newton release. Currently zero downtime update process [1] works
well on liberty but it will be affected by several changes that are targeted
for newton release.

This blueprint should ensure that updates won't be affected by these changes
and we'll track any necessary work.

Changes that need to be evaluated:

* Composable services within roles [2][3] where the effort is to split
monolithic templates into separate templates. This might affect update
process as we should restart only services that were deployed on the
controller.

* Support a new HA architecture [4] where the effort is to implement
next generation of HA architecture. We have to ensure that undercloud will
manage duality of deployments as it should support also previous version of
openstack.

[1] - http://tripleo.org/post_deployment/package_update.html
[2] - https://etherpad.openstack.org/p/tripleo-composable-services
[3] - https://blueprints.launchpad.net/tripleo/+spec/composable-services-within-roles
[4] - https://review.openstack.org/#/c/299628