Centralize Config Options

Heavily inspired by https://specs.openstack.org/openstack/nova-specs/specs/mitaka/implemented/centralize-config-options.html

Here is a non-exhaustive the missing conf modules to be implemented:

- api
- decision_engine
- applier
- planner
- nova_client
- glance_client
- cinder_client
- ceilometer_client
- neutron_client
- clients_auth

Here are the files which contain the configuration options that would have to be moved:

- watcher/api/acl.py
- watcher/api/app.py
- watcher/applier/manager.py
- watcher/cmd/dbmanage.py
- watcher/common/clients.py
- watcher/common/exception.py
- watcher/common/paths.py
- watcher/common/service.py
- watcher/common/utils.py
- watcher/db/sqlalchemy/models.py
- watcher/decision_engine/manager.py
- watcher/decision_engine/audit/continuous.py
- watcher/decision_engine/planner/manager.py