Even though the feature is technically virt driver agnostic,
the plumbing happens through the virt drivers, so the feature
is only supported by certain virt drivers (libvirt only at
the time of this patch). So this adds a section to the feature
support matrix about the trusted certs validation feature.
Also updates the certificate validation user docs based on
the nova boot --trusted-image-certificate-id option name
in the dependent python-novaclient change.
Depends-On: https://review.openstack.org/500396/
Related to blueprint nova-validate-certificates
Change-Id: Ic5cb4a98c73cc404c7033cf183f25a97aba3c994
ZhongShengping