[placement] Update the placement deployment instructions
[placement] Update the placement deployment instructions

The web-server-deployment section of placement.rst has been updated
to provide additional links to information and reflect the fact that
placement is now using uwsgi and mod_proxy_uwsgi in devstack. This
does not provide a full set of installations instructions. This is
somewhat intentional:

* there are many ways to deploy a wsgi application and we'd like the
  packagers and deployers to choose a way that works best for them,
  not impose one, and there's no way for us to document them all. It
  is better to point to resources that explain some of the options
  and allow people to inform themselves so they can make informed
  choices
* we're no longer that keen on the mod_wsgi, but it tends to be the
  easiest to document (fewer moving parts)
* the uwsgi+systemd method in devstack, while great, is abstracted
  enough that the moving parts are not entirely visible and is one
  of several ways for that scenario

Change-Id: Ief07c313e012df63558de632047258e8e11736c1
Related-Bug: #1692375

klmitch/nova,gooddata/openstack-nova,openstack/nova,gooddata/openstack-nova,mahak/nova,rahulunair/nova,mahak/nova,mikalstill/nova,rahulunair/nova,klmitch/nova,openstack/nova,klmitch/nova,phenoxim/nova,mikalstill/nova,gooddata/openstack-nova,gooddata/openstack-nova,phenoxim/nova,mahak/nova,openstack/nova,klmitch/nova,mikalstill/nova,rahulunair/nova