Add COMPUTE_TRUSTED_CERTS trait

This mirrors the compute driver "supports_trusted_certs"
capability added to nova in change:

  Ie3130e104d7ca80289f1bd9f0fee9a7a198c263c

This is something validated during server creation
and if the user requests trusted image cert validation
but the driver does not support it, the create request
on that host fails. We eventually want to publish these
driver capabilities as traits so we can optimize scheduling
to avoid those types of capabilities-based build failures:

  I15364d37fb7426f4eec00ca4eaf99bec50e964b6

Related to blueprint nova-validate-certificates

Change-Id: I0a67ce94b03b5707802935ddd36aedc622fe12fe

os_traits/compute/__init__.py

@@ -15,4 +15,6 @@
 TRAITS = [
     # The virt driver supports associating a tag with a device *at boot time*
     'DEVICE_TAGGING',
+    # The virt driver supports trusted image certificate validation
+    'TRUSTED_CERTS',
 ]