Fix assert test error under py3.6
Fix assert test error under py3.6

The function __str__ of class subprocess.CalledProcessError
is different between py3.6 and lower python version.

        py3.6: Command '%s' returned non-zero exit status %d.
lower version: Command '%s' returned non-zero exit status %d

There is a . in py3.6.

This patch fix the assert error under py3.6

Change-Id: I19ae5711ed7440791583940fa4a8fb770dcdc933
Closes-bug: #1751551

openstack/keystone,openstack/keystone,mahak/keystone,mahak/keystone,mahak/keystone,openstack/keystone