Add enable_certificate_validation config option

This patch adds enable_certificate_validation config option. This option
can be used to skip tests that rely on image signature certificate
validation being enabled on the test environment
(test_signed_image_invalid_cert_boot_failure).

Change-Id: Id4134a2e87378487baa9e3d5f49e7ded48daa765
diff --git a/barbican_tempest_plugin/config.py b/barbican_tempest_plugin/config.py
index da78d15..76999b9 100644
--- a/barbican_tempest_plugin/config.py
+++ b/barbican_tempest_plugin/config.py
@@ -90,6 +90,10 @@
                 default=True,
                 help="Does the test environment enforce glance image "
                      "verification?"),
+    cfg.BoolOpt('certificate_validation',
+                default=True,
+                help="Does the test environment enforce image signature"
+                     "certificate validation?")
 ]
 
 barbican_rbac_scope_verification_group = cfg.OptGroup(