WSGI: make it work, and test it with acceptance

WSGI: make it work, and test it with acceptance

* Finalize httpd service name support in API class following
  puppet-keystone model.
* Implement unit tests.
* Implement separated acceptance tests that deploy Ceilometer API in
  WSGI.

This patch aims to finalize and validate we can deploy Ceilometer API in
WSGI with puppet-ceilometer.
Also that we can update from eventlet to WSGI (it's tested in our beaker jobs).

Closes-bug: #1481532
Change-Id: I3695acf5522457837e079c36d68a0e35dbf6b72c
(cherry picked from commit 257a60ce79f00f96239d06d84c9660f14a31bd68)