galera: Fix instance name in master_exists()

Under certain circumstances $OCF_RESOURCE_INSTANCE as passed to the
resource agent contains and instance number (e.g. "galera:0"). This
happens e.g. when using "crm_resource --force-stop/--force-start". In
those cases "master_exists()" doesn't work correctly.

Use $INSTANCE_ATTR_NAME which is set in "mysql-common.sh", based on
$OCF_RESOURCE_INSTANCE with the instance number stripped.