Add trust level cache to trusted_filter

    Currently each time the trusted filter is called to check host_pass, it
polls Open Attestation Service to get the trusted level for the host. This
solution is not good on scalability.
    With a cache for the host trust level, trusted filter don't need to
consult OAT service if the cache is still valid, thus improves the
scalability. The cache is setup when the first time the filter consulting
the OAT service.
    The trusted_filters are reinitialized for each VM that is
scheduled, thus the cache is valid only for one VM instance scheduing.

    Implements blueprint trusted-filter-cache

Change-Id: I613da53876e6cd548566e27ab1693287023e9861
Signed-off-by: Yunhong, Jiang <yunhong.jiang@intel.com>
Merge remote-tracking branch 'citrix/master' into build-temp