galera: recover from joining a non existing cluster

galera being a M/S resource, the resource agent decides
when and how to promote a resource based on the current
state of the galera cluster. If there's no cluster,
a resource is promoted as the bootstrap node. Otherwise
it is promoted as a joiner node.

There can be some time between the moment when a node is
promoted and when the promote operation effectively
takes place. So if a node is promoted for joining a cluster,
all the running galera nodes are stopped before the promote
operation start, the joining node won't be able to join the
cluster, and it can't bootstrap a new one either because it
doesn't have the most recent copy of the DB.

In that case, do not make the promotion fail, and force
a demotion instead. This ensures that a normal bootstrap
election will take place eventually, without blocking
the joining node due to a failed promotion.