GDA-5448 Make Ldap{Authenticator,Authoriser} more resilient to LDAP server failure

Remove the "gda.jython.authenticator.ldap.url" property from your GDA config,
and altfed.cclrc.ac.uk will be used. That hostname currently resolves to four
servers; each one will be tried before giving up.

The "gda.jython.authenticator.ldap.hosts" can be set to a space-separated list
of hostnames and/or IP addresses. This shouldn't be necessary at Diamond.