kamailio.cfg: test if DBURL is already defined before defining it

- allow to set it via command line with -A DBURL='...'

etc/kamailio.cfg

@@ -105,8 +105,10 @@
 #!ifdef WITH_MYSQL
 # - database URL - used to connect to database server by modules such
 #       as: auth_db, acc, usrloc, a.s.o.
+#!ifndef DBURL
 #!define DBURL "mysql://openser:openserrw@localhost/openser"
 #!endif
+#!endif
 #!ifdef WITH_MULTIDOMAIN
 # - the value for 'use_domain' parameters
 #!define MULTIDOMAIN 1