Add roles to RequestContext.to_dict if they're not there

Old versions of oslo_context do not include rules in to_dict()
However roles are required for newer versions of oslo_policy to operate
and resolve 'role:xxx' rules.

Since minimal version for oslo_context is 0.2.0 and commit, that adds
roles has been added only in 2.2.0 we should support old versions of
oslo_context with newer version.

Change-Id: If35726613bec5d342bad72b542215ec8e5c096a2
Closes-Bug: #1552291

murano/context.py

@@ -34,3 +34,8 @@ class RequestContext(context.RequestContext):
         self.is_admin = is_admin
         if self.is_admin is None:
             self.is_admin = policy.check_is_admin(self)
+
+    def to_dict(self):
+        d = super(RequestContext, self).to_dict()
+        d.setdefault('roles', self.roles)
+        return d

releasenotes/notes/roles-for-requestcontext-43d32d88c3eaaa95.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - RequestContext now serialises it's roles. This should allow murano to
+    work correctly (and allow rules like "role:xxx" in policy.json)
+    when using oslo.context prior to 2.2.0 and oslo.policy