Use RBAC policy to determine if context is admin.

Fixes bug 1152716

If the context roles do not match the configured admin_role,
fall back to determining if admin via the "context_is_admin"
RBAC policy rule (for consistency with the approach used by
the other projects).

Note this requires that the "context_is_admin" rule *must*
be set in the policy.json if the out-of-the-box default rule
is used (as this default is so open, the net effect of omitting
the "context_is_admin" rule is for every request to acquire
admin status).

Change-Id: Ide2cf604b48f24bd759ce2d65091ff546cd9d22e