Galera: Allow master promotion with lower number of clones

When a node is down for maintenance and the galera resource agent is checking last-commits, it will wait for ALL nodes to report status, which will never happen unless that node is taken out of maintenance.

Propose to add an optional meta variable with the number of minimum instances to be up before master selection starts, or just a flag to start master election "now", with the last-commits that have been recorded so far.

Start by locating the Galera resource agent and tracing its last-commits check while a node is in maintenance. Define whether completion means supporting a minimum active-instance setting, an immediate-election flag, or both, then verify that master promotion can use the commits recorded so far without waiting for every node.

Written by the indexing model from the issue text.