galera: allow joiner to report non-Primary during initial IST

It seems that with recent galera versions, when a galera node
joins a cluster, there is a small time window where the node is
connected to the primary component of the galera cluster, but it
might still be preparing its IST. During this time, it can report
itself as being 'not ready' and in 'non-primary' state.

Update the galera resource agent to allow the node to be in
non-primary state, but only if running a "promote" operation. Any
network partition during the promotion will be caught by the
promote timeout.

In reworking the promotion code, we move the check for primary
partition into the "galera_monitor" function. The check works
as before for regular "monitor" or "probe" operations.

Related-Bug: rhbz#2255414