MXS-1335: Disable root_node_as_master by default

Enabling the option hinders the use of maintenance mode with the root
master node in most use-cases.

This behavior occurs due to the fact that the maintenance mode causes a
server to be treted as if it was down. The Galera monitor waits for the
cluster to reorganize before assigning a new master node. This is correct
(but very unexpected) behavior for single instance use-cases.

Documentation/Monitors/Galera-Monitor.md

@@ -65,8 +65,9 @@ use_priority=true
 ### `root_node_as_master`
 
 This option controls whether the write master Galera node requires a
-_wsrep_local_index_ value of 0. This option is enabled by default and was
+_wsrep_local_index_ value of 0. This option was introduced in MaxScale 2.1.0 and
-introduced in MaxScale 2.1.0.
+it is disabled by default in versions 2.1.5 and newer. In versions 2.1.4 and
+older, the option was enabled by default.
 
 A Galera cluster will always have a node which has a _wsrep_local_index_ value
 of 0. Based on this information, multiple MaxScale instances can always pick the

Documentation/Release-Notes/MaxScale-2.1.5-Release-Notes.md

@@ -26,6 +26,12 @@ and the rest of the certificates on the file are stored in the chain store.
 
 This change should not cause any changes in MaxScale's behavior.
 
+### `root_node_as_master`
+
+The galeramon parameter `root_node_as_master` is now disabled by default. The
+option should be enabled when it is of great importance to know that all
+MaxScale instances treat a shared Galera cluster in the same way.
+
 ### Schemarouter
 
 Starting with MaxScale 2.1.5, the _schemarouter_ will prioritize the current
@@ -57,6 +63,7 @@ will still go to the central database.
 [Here is a list of bugs fixed in MaxScale 2.1.5.]
 (https://jira.mariadb.org/issues/?jql=project%20%3D%20MXS%20AND%20issuetype%20%3D%20Bug%20AND%20status%20%3D%20Closed%20AND%20fixVersion%20%3D%202.1.5)
 
+* [MXS-1335](https://jira.mariadb.org/browse/MXS-1335) root_node_as_master should not be enabled by default
 * [MXS-1330](https://jira.mariadb.org/browse/MXS-1330) insertstream attempts to parse all buffers
 * [MXS-1329](https://jira.mariadb.org/browse/MXS-1329) Using filters with SSL and keep alive can cause errors
 * [MXS-1328](https://jira.mariadb.org/browse/MXS-1328) Strange behavior with routes between master / slaves

server/modules/monitor/galeramon/galeramon.c

@@ -93,7 +93,7 @@ MXS_MODULE* MXS_CREATE_MODULE()
             {"disable_master_failback", MXS_MODULE_PARAM_BOOL, "false"},
             {"available_when_donor", MXS_MODULE_PARAM_BOOL, "false"},
             {"disable_master_role_setting", MXS_MODULE_PARAM_BOOL, "false"},
-            {"root_node_as_master", MXS_MODULE_PARAM_BOOL, "true"},
+            {"root_node_as_master", MXS_MODULE_PARAM_BOOL, "false"},
             {"use_priority", MXS_MODULE_PARAM_BOOL, "false"},
             {
                 "script",