It's not missing: it does not exist upstream.

To get into an RH build it first needs to go there.

Verified as follows,

[root@artyom-rhevm33 ~]# yum list installed | grep sos
sos.noarch                           3.2-16.el7ost.5         @rhelosp-7.0-puddle
[root@artyom-rhevm33 ~]# 
[root@artyom-rhevm33 ~]# 
[root@artyom-rhevm33 ~]# rpm -ql sos | grep instack
/usr/lib/python2.7/site-packages/sos/plugins/openstack_instack.py
/usr/lib/python2.7/site-packages/sos/plugins/openstack_instack.pyc
/usr/lib/python2.7/site-packages/sos/plugins/openstack_instack.pyo
[root@artyom-rhevm33 ~]#

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHBA-2015:1875