Fix roles attribute for barbican request context

The oslo_context.context.RequestContext was updated in oslo_context 2.2.0
to have a roles attribute. This caused the barbican request context to
have an empty roles attribute.
The 'inspect' library is used to make sure backwards compatibility since
the current requirements for oslo_context is >= 0.2.0.

Change-Id: Iafc67a6f42e46da998eaf2f5199c47f6beaf345e

barbican/context.py

@@ -13,6 +13,7 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+import inspect
 import oslo_context
 from oslo_policy import policy
 
@@ -34,8 +35,17 @@ class RequestContext(oslo_context.context.RequestContext):
         if project:
             kwargs['tenant'] = project
         self.project = project
-        self.roles = roles or []
         self.policy_enforcer = policy_enforcer or policy.Enforcer(CONF)
+
+        # NOTE(edtubill): oslo_context 2.2.0 now has a roles attribute in
+        # the RequestContext. This will make sure of backwards compatibility
+        # with past oslo_context versions.
+        argspec = inspect.getargspec(super(RequestContext, self).__init__)
+        if 'roles' in argspec.args:
+            kwargs['roles'] = roles
+        else:
+            self.roles = roles or []
+
         super(RequestContext, self).__init__(**kwargs)
 
     def to_dict(self):