Use config-generator.conf to generate config.sample file

[Vincent Françoise]

We hard code configure options in tox.ini to generate Watcher config sample file, and we keep this config sample file in our repository.

It is inconvenient for developer if the add/remove/change some configuration options cause they need to take care about the
config.sample file.

So the objective it to do it the way Magnum dealt with it: https://review.openstack.org/#/c/272872/

We should also add a new page to the .rst doc which would display a generated version of the configuration file via a custom directive.

---
Related to https://bugs.launchpad.net/magnum/+bug/1538374

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/276757

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/276757
Committed: https://git.openstack.org/cgit/openstack/watcher/commit/?id=c9e0dfd3f59ec45284f663da19e589fdea39b161
Submitter: Jenkins
Branch: master

commit c9e0dfd3f59ec45284f663da19e589fdea39b161
Author: David TARDIVEL <email address hidden>
Date: Fri Feb 5 15:16:50 2016 +0100

    Remove the watcher sample configuration file

    Watcher sample configuration file groups parameters from
    various projects and the watcher project ones. This makes it
    tricky to review updates on configuration parameters.

    It is inconvenient for developer if the add/remove/change some
    configuration options cause they need to take care about the
    config.sample file.

    The sample configuration file should be available into HTML doc.

    This patchset:
    . removes the file /etc/watcher/watcher.conf.sample
    . adds an admin script tool to be able to built it, by using tox
    . includes a new section 'Watcher sample configuration files' into
      the doc source files
    . uses sphinx extension oslo_config.sphinxgenconfig

    Change-Id: If2180de3614663f9cbc5396961a8d2175e28e315
    Closes-Bug: #1541734