Fix: controld: leave xml-src attribute empty when no DC selected #2902

Right after starting the cluster the DC is offline for short time,
and if you call 'crmadmin --dc_lookup' during this time, it would hang.
This change leaves the xml-src attribute that the crmadmin receives from
the pacemaker-controld empty, so that crmadmin knows the DC was not
selected.
ref: https://projects.clusterlabs.org/T735