During an upgrade, we run the upgrade in serial, and we attempt to
bootstrap the cluster from the "galera_server_bootstrap_node". This
causes issues when there is an existing cluster that is still up.
To avoid this, we can simply attempt to start the cluster normally, if
it fails to start, and join the existing cluster, we can bootstrap as
usual.
Additionally, we can make the bootstrap slightly more efficient by only
running it against the bootstrap host and splitting out the
"galera_upgrade_post.yml" tasks.
Finally - run the upgrade in serial which mirrors our approach in the
integrated repo.
Change-Id: Ic4d69f0fa75c1eea81d10a76cca9a8d9c3822094
Closes-Bug: #1667103
(cherry picked from commit db64210b12)
0508515d25