Fix rbac_rule_validation log statement raises TypeError

If an OverPermission error occurs, then the LOG.error statement
that should execute throws a TypeError due to a minor syntax issue.

Example: http://logs.openstack.org/31/466331/6/check/gate-tempest-dsvm-patrole-member-ubuntu-xenial/dc38b01/console.html

Change-Id: I3f7fde708521cfd200fd0835a6f2d697ed83b08b
Closes-Bug: #1693297
2 files changed