Remove rackspace/centos7 work-around

The current centos 7 images from rackspace do not have cloud-init
installed via pip, so this is no longer needed.  This work around is
actually a little wrong too, see the bug.

Change-Id: I6642593ca2248d9e01f653cc8e70c70ba8372e50
Partial-bug: #1393073
Merge "Remove rackspace/centos7 work-around"