Prevent multiple MySQL masters

When CONTROLSCALE is set to >1 and no mysql.nodes are configured, the
previous behavior resulted in MySQL being configured as a master node
on all control nodes.

This change detects that condition and returns an error to
os-refresh-config if encountered.

Change-Id: I7837a8496a316775b6f0a399e6c237a25a9a24d4
Closes-Bug: 1336915
Merge "Prevent multiple MySQL masters"