Implement the certificate_utils module. The module's verify_certificate
method can be applied to the creation or rebuild of an instance. It is
triggered by one of two ways:
1) The enable_certificate_validation configuration option is set to
True in Nova's glance configuration (alongside the
verify_glance_signatures option also set to True)
2) A list of trusted certificate IDs is provided
Change-Id: I0ae2dbf66241207a425bf7d0fc02a4d2e2dea409
Implements: blueprint nova-validate-certificates
595ea73c62