Gnocchi API coundn't be use if we don't install and configure uwsgi
or other HTTP daemon. And gnocchi install guide didn't describe
those.
Add the describe of installing uWSGI and the brief relationship
between uWSGI and Gnocchi API. And tell the reader that they can
also use Apache httpd and mod_wsgi, or any other HTTP daemon to
run Gnocchi API.
Closes-Bug: #1750933
Change-Id: Iba8c1fe7a64569de4ca366251c495731a8d7a779