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

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

In this patchset, follow changes has been introduced:
1. Remove Cache, and add  hashmap<String, AttestationResultEnum> which for each VDS hostname it maps to the attested status.
2. Add initialization mechanism when engine reboot and set up a thread to support 2-phase aggregated attestation.
3. Bug fixing and code cleanup.

Change-Id: I8ce3448a821c74521d277f92f2c8d63ba0accfed
Signed-off-by: Jimmy <gang.wei@intel.com>
Signed-off-by: Zhang Lijuan <lijuan.zhang@intel.com>
Signed-off-by: Quan Xu <quan.xu@intel.com>
Signed-off-by: Ofri Masad <omasad@redhat.com>
Signed-off-by: Dave Chen <wei.d.chen@intel.com>