Respect DB_IP and DB_PORT in mongodb tests.

It is usually preferred to run tests on a test-local database with
dedicated mongod instance rather than using the system-wide database.
In order to support that, an easy way of providing the proper port
is handy.

For example, pymongo test suite uses DB_IP and DB_PORT environment
variables to allow providing an alternate mongod instance. Using
the same variable names in IPython's test suite seems a good idea.

IPython/parallel/tests/test_mongodb.py

@@ -1,44 +1,52 b''
 """Tests for mongodb backend
 
 Authors:
 
 * Min RK
 """
 
 #-------------------------------------------------------------------------------
 #  Copyright (C) 2011  The IPython Development Team
 #
 #  Distributed under the terms of the BSD License.  The full license is in
 #  the file COPYING, distributed as part of this software.
 #-------------------------------------------------------------------------------
 
 #-------------------------------------------------------------------------------
 # Imports
 #-------------------------------------------------------------------------------
 
+import os
+
 from unittest import TestCase
 
 from nose import SkipTest
 
 from pymongo import Connection
 from IPython.parallel.controller.mongodb import MongoDB
 
 from . import test_db
 
+conn_kwargs = {}
+if 'DB_IP' in os.environ:
+    conn_kwargs['host'] = os.environ['DB_IP']
+if 'DB_PORT' in os.environ:
+    conn_kwargs['port'] = int(os.environ['DB_PORT'])
+
 try:
-    c = Connection()
+    c = Connection(**conn_kwargs)
 except Exception:
     c=None
 
 class TestMongoBackend(test_db.TaskDBTest, TestCase):
     """MongoDB backend tests"""
 
     def create_db(self):
         try:
             return MongoDB(database='iptestdb', _connection=c)
         except Exception:
             raise SkipTest("Couldn't connect to mongodb")
 
 def teardown(self):
     if c is not None:
         c.drop_database('iptestdb')