OVN-K alerts: add ovn db cluster member error

OVN northbound and southbound Databases both have RAFT clusters. See here for more details: https://web.stanford.edu/~ouster/cgi-bin/cs190-winter21/lecture.php?topic=raft

This story is to create alerts based on metric ovn_db_cluster_server_status.

ovn_db_cluster_server_status gauge

• Description
• A metric with a constant '1' value labeled by database name, cluster uuid, server uuid server status. The label ‘server_status’ which represents the RAFT status of the db, can be: ‘joining cluster’, ‘leaving cluster’, ‘left cluster’, ‘failed’, ‘disconnected from the cluster (election timeout)’ or ‘cluster member’.

• Normal/Expected values
• For each database, (nb or sb), for the vast majority of time, there is an entry with label ‘server_status’ is ‘cluster member’

PR that includes the commit for this change: https://github.com/openshift/cluster-network-operator/pull/1526

See commit titled: OVN-K alerts: add ovn db cluster member error

See code for two alerts which will fire if there is a cluster member error.