Make possible to override default namespace (/service/) from a config file

If the namespace is not specified in a config file /service/ would be
used.
Also it's possible to use just '/' as a namespace. It means we would
have following structure:
  /scope1
  /scope2
  ...

patroni/dcs.py

@@ -122,8 +122,8 @@ class AbstractDCS:
             i.e.: `zookeeper` for zookeeper, `etcd` for etcd, etc...
         """
         self._name = name
-        self._scope = config['scope']
-        self._base_path = '/service/' + self._scope
+        self._namespace = '/{}'.format(config.get('namespace', '/service/').strip('/'))
+        self._base_path = '/'.join([self._namespace, config['scope']])
 
         self._cluster = None
         self._cluster_thread_lock = Lock()

tests/test_etcd.py

@@ -80,7 +80,7 @@ def etcd_watch(key, index=None, timeout=None, recursive=None):
 def etcd_write(key, value, **kwargs):
     if key == '/service/exists/leader':
         raise etcd.EtcdAlreadyExist
-    if key == '/service/test/leader':
+    if key == '/service/test/leader' or key == '/patroni/test/leader':
         if kwargs.get('prevValue', None) == 'foo' or not kwargs.get('prevExist', True):
             return True
     raise etcd.EtcdException
@@ -204,11 +204,14 @@ class TestEtcd(unittest.TestCase):
     def setUp(self):
         with patch.object(Client, 'machines') as mock_machines:
             mock_machines.__get__ = Mock(return_value=['http://localhost:2379', 'http://localhost:4001'])
-            self.etcd = Etcd('foo', {'ttl': 30, 'host': 'localhost:2379', 'scope': 'test'})
+            self.etcd = Etcd('foo', {'namespace': '/patroni/', 'ttl': 30, 'host': 'localhost:2379', 'scope': 'test'})
             self.etcd.client.write = etcd_write
             self.etcd.client.read = etcd_read
             self.etcd.client.delete = Mock(side_effect=etcd.EtcdException())
 
+    def test_base_path(self):
+        self.assertEquals(self.etcd._base_path, '/patroni/test')
+
     @patch('dns.resolver.query', dns_query)
     def test_get_etcd_client(self):
         with patch.object(etcd.Client, 'machines') as mock_machines: