There seems to be a bug regarding the interaction between the Red Hat
based images and docker exec, where output is missed when attaching a
tty. This can be replicated using the following:

$ docker run -d --name test centos /bin/sleep infinity
$ docker exec -t test echo hi

Repeat the second command over and over, "hi" will only be printed out
every couple of runs.

This affects the keystone register task as sometimes it will not print
it's final json result to stdout, causing ansible to fall over with a
difficult to diagnose error (despite the register having run
successfully).

Disabling the tty fixes this for me, it should not be needed in this
case regardless.

Change-Id: Ie7eb7c01c34ee3c59bd843651195fbcb7259d2c8
Closes-Bug: #1572082