fix mysql_info crash if PUBLIC role is used on MariaDB 10.11+ (#712)

* tests: grant a privilege to the PUBLIC role to force the module to fail

* skip the PUBLIC role from the users migration example

* Cut the 3rd matching group as it fails to match roles without quotes

The group purpose was to match the closing quotes. We don't need the
value after the TO. So we can ignore this entirely.

* add MariaDB role detection

* Add changelog fragment

* doc: add notes that users_info doesn't support MariaDB roles.

plugins/modules/mysql_info.py

@@ -50,6 +50,7 @@ notes:
 - Compatible with MariaDB or MySQL.
 - Calculating the size of a database might be slow, depending on the number and size of tables in it.
   To avoid this, use I(exclude_fields=db_size).
+- filters C(users_info) doesn't support MariaDB roles.
 
 attributes:
   check_mode:
@@ -161,6 +162,7 @@ EXAMPLES = r'''
       - item.name != 'root'  # In case you don't want to import admin accounts
       - item.name != 'mariadb.sys'
       - item.name != 'mysql'
+      - item.name != 'PUBLIC'  # MariaDB roles are not supported
 '''
 
 RETURN = r'''
@@ -606,7 +608,9 @@ class MySQL_Info(object):
             user = line['User']
             host = line['Host']
 
-            user_priv = privileges_get(self.cursor, user, host)
+            # MariaDB roles have no host
+            is_role = self.server_implementation == 'mariadb' and not host
+            user_priv = privileges_get(self.cursor, user, host, maria_role=is_role)
 
             if not user_priv:
                 self.module.warn("No privileges found for %s on host %s" % (user, host))