Add support to change root password

Change the configured root's password in percona and update the leader
settings.

Change-Id: I7168a96cac7a3b4df7fcfa1afc6f35076748578b
Partial-Bug: 1436093

hooks/percona_hooks.py

@@ -105,6 +105,7 @@ from percona_utils import (
     pxc_installed,
     update_bootstrap_uuid,
     LeaderNoBootstrapUUIDError,
+    update_root_password,
 )
 
 
@@ -338,6 +339,11 @@ def config_changed():
 
     open_port(DEFAULT_MYSQL_PORT)
 
+    # the password needs to be updated only if the node was already
+    # bootstrapped
+    if bootstrapped:
+        update_root_password()
+
 
 @hooks.hook('cluster-relation-joined')
 def cluster_joined():