engine:Trusted Compute Pools - Open Attestation integration with oVirt engine

Detailed description: http://wiki.ovirt.org/Trusted_compute_pools

Add rule to canDoAction to block setting of cluster as trusted when Attestation Server is not defined properly.

Change-Id: I1dd7da6ed6ff33cb8927fb8853eb9799cf178190
Signed-off-by: Jimmy <gang.wei@intel.com>
Signed-off-by: Emily <lijuan.zhang@intel.com>