FR: Add cluster synchronization support for global 'ldap-%' variables and 'mysql_ldap_mapping' provided by 'ldap_auth_plugin'

[JavierJF]

• A clear description of the issue

Right now Cluster isn't aware of new introduced global 'ldab-%' variables, or the 'mysql_ldap_mapping' table introduced by 'ldap_auth_plugin'. Support should be added to, in case of plugin being found, syncing both of them:

• 'mysql_ldap_mapping' should be sync as part as the 'mysql_users' module.
• New sync module should be created for 'ldab-%' variables, with it's own checksum.
• New metrics should be introduced for providing insights about the synchronization as for the other modules.