Configure T1 disk sites overrride

Allow a configurable list of SEs where the Disk node
should have priority over the Buffer, this is used
by T1 sites where the Disk instance is commissioned
and uses the same SE endpoint as the Buffer instance.

It defaults to the SE of CNAF and the Disk SE of RAL. It can be configured
as:

config.PhEDExInjector.diskSites = ["storm-fe-cms.cr.cnaf.infn.it","srm-cms-disk.gridpp.rl.ac.uk"]