Fix string format used to report content validation errors
Fix string format used to report content validation errors

The format used ('{!s}') was not compatible with Python 2.6. Replaced
this format with ('{0}').

Change-Id: Iab252cf867f16e4da354d9634d8ae4712fefb77e
Closes-Bug: #1291482

openstack/barbican,openstack/barbican,MCDong/barbican,jmvrbanac/barbican,jmvrbanac/barbican,MCDong/barbican,cneill/barbican,cloudkeep/barbican,cneill/barbican,cloudkeep/barbican