XenServer: Enable injection of XVA and ISO urls

This will enable the flexible configuration of the XVA and the URL
through nodepool, thus it's not needed to push an infra change for
changing those values. Also if you do not have those values configured,
sensible defaults are provided.

Change-Id: Ie2be50f842b4220d4e7f9d8e830d3df3262a6353
Merge "XenServer: Enable injection of XVA and ISO urls"