Don't allow both two_node and auto_tie_breaker in corosync.conf

The two_node and auto_tie_breaker options are incompatible as they
specify conflicting methods of determining the quorate half of a cluster
partition.

This patch detects this error in corosync.conf, issues a message and
disables two_node if auto_tie_breaker is present.

Signed-Off-By: Christine Caulfield <ccaulfie@redhat.com>
Reviewed-by: Jan Friesse <jfriesse@redhat.com>