[install-guide] Update Obs-only comment
[install-guide] Update Obs-only comment

The original patch was pushed with this comment for
all distros. This was incorrect. It should be noted
that this change should only be required/noted by
the SLES users.

See the bug report for further explanation.

Change-Id: I06343a4b3fb94d544a3d6332aac79760944d8779
Closes-bug: 1635546

openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals