[Docs] need doc for ceph upgrade ocata to pike

[John Fulton]

The TripleO doc doc/source/install/post_deployment/upgrade.rst states [1] the following:

        It is not yet possible to migrate an existing puppet-ceph
        deployment to a ceph-ansible deployment. Only new deployments
        are currently possible with ceph-ansible.

This is no longer true so it should be updated to cover how to do the migrations.

[1] https://github.com/openstack/tripleo-docs/commit/69b6c94dbae28ca1caa7f17b65330f97c012581f#diff-c4ce91eaa1834376a067ae51d369ed9e

[John Fulton (jfulton-org)]

https://review.openstack.org/#/c/511635/2

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/511635
Committed: https://git.openstack.org/cgit/openstack/tripleo-docs/commit/?id=31f13bff7ad1160f29ae26752de255f9db9af3f5
Submitter: Zuul
Branch: master

commit 31f13bff7ad1160f29ae26752de255f9db9af3f5
Author: Giulio Fidente <email address hidden>
Date: Fri Oct 13 00:28:18 2017 +0200

    Update upgrade documentation with the Ceph related steps

    Adds in the upgrade guide the notes and additional steps necessary
    to migrate the Ceph cluster to containers.

    Change-Id: I923e8aa34d542e6f503baf1d776a5c3fdd8a2fda
    Closes-Bug: #1716497