Run Watcher-API behind mod-wsgi

This patch adds support to run watcher-api
with mod-wsgi. It provides
1. wsgi app script files, to run watcher-api under apache.
2. updated devstack plugin to run watcher-api default with
   mod-wsgi.
3. Document to deploy watcher-api behind wsgi.

Change-Id: I8f1026f0b09fd774376220c2d117ee66f72421b8
Closes-Bug: #1675376

watcher/api/app.wsgi

@@ -0,0 +1,40 @@
+# -*- mode: python -*-
+# -*- encoding: utf-8 -*-
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may
+# not use this file except in compliance with the License. You may obtain
+# a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+# License for the specific language governing permissions and limitations
+# under the License.
+"""
+Use this file for deploying the API service under Apache2 mod_wsgi.
+"""
+
+import sys
+
+from oslo_config import cfg
+import oslo_i18n as i18n
+from oslo_log import log
+
+from watcher.api import app
+from watcher.common import service
+
+
+CONF = cfg.CONF
+
+i18n.install('watcher')
+
+service.prepare_service(sys.argv)
+
+LOG = log.getLogger(__name__)
+LOG.debug("Configuration:")
+CONF.log_opt_values(LOG, log.DEBUG)
+
+application = app.VersionSelectorApplication()
+