dcache-core, alarms: restore xml connector to initialization properties

A recent refactoring (http://rb.dcache.org/r/6541) of the connector initialization for the alarms appender inadvertently eliminated the url property for the xml datanucleus plugin.

This patch restores it.

(Note: This is my fault, as I acked that patch and should have noticed it).

Target: master
Require-book: no
Require-notes: no
Patch: http://rb.dcache.org/r/6571
Acked-by: Paul