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

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

Fix parser function to parse complex response string returned from OAT server in AttestationService.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1107517

Change-Id: I2f541b004ba383a3098cf7fbfca7a000b11b750f
Signed-off-by: Emily <lijuan.zhang@intel.com>
Signed-off-by: Jimmy <gang.wei@intel.com>
Signed-off-by: Dave <wei.d.chen@intel.com>
Signed-off-by: Emily <lijuan.zhang@intel.com>