Add serf inventory plugin

Add inventory plugin for [Serf](https://serfdom.io/).
Requires [`serfclient` Python module](https://pypi.python.org/pypi/serfclient).

plugins/inventory/serf.py

+#!/usr/bin/env python
+
+# (c) 2015, Marc Abramowitz <marca@surveymonkey.com>
+#
+# This file is part of Ansible.
+#
+# Ansible is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# Ansible is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with Ansible. If not, see <http://www.gnu.org/licenses/>.
+
+# Dynamic inventory script which lets you use nodes discovered by Serf
+# (https://serfdom.io/).
+#
+# Requires host to be a member of a Serf cluster and the `serfclient` Python
+# module from https://pypi.python.org/pypi/serfclient
+
+import argparse
+import sys
+
+# https://pypi.python.org/pypi/serfclient
+from serfclient.client import SerfClient
+
+try:
+    import json
+except ImportError:
+    import simplejson as json
+
+_key = 'serf'
+
+
+def get_serf_members_data():
+    serf = SerfClient()
+    return serf.members().body['Members']
+
+
+def get_nodes(data):
+    return [node['Name'] for node in data]
+
+
+def get_meta(data):
+    meta = {'hostvars': {}}
+    for node in data:
+        meta['hostvars'][node['Name']] = node['Tags']
+    return meta
+
+
+def print_list():
+    data = get_serf_members_data()
+    nodes = get_nodes(data)
+    meta = get_meta(data)
+    print(json.dumps({_key: nodes, '_meta': meta}))
+
+
+def print_host(host):
+    data = get_serf_members_data()
+    meta = get_meta(data)
+    print(json.dumps(meta['hostvars'][host]))
+
+
+def get_args(args_list):
+    parser = argparse.ArgumentParser(
+        description='ansible inventory script reading from serf cluster')
+    mutex_group = parser.add_mutually_exclusive_group(required=True)
+    help_list = 'list all hosts from serf cluster'
+    mutex_group.add_argument('--list', action='store_true', help=help_list)
+    help_host = 'display variables for a host'
+    mutex_group.add_argument('--host', help=help_host)
+    return parser.parse_args(args_list)
+
+
+def main(args_list):
+    args = get_args(args_list)
+    if args.list:
+        print_list()
+    if args.host:
+        print_host(args.host)
+
+
+if __name__ == '__main__':
+    main(sys.argv[1:])