Add default mongod URI to README
Add default mongod URI to README

It could be helpful to add an example mongod URI to the instructions
in README for running a local instance of Marconi. Then the setup
procedures can be more smoother. Add an example mongod URI with
lookback address(127.0.0.1) and default mongod port(27017).

Closes-Bug: #1289355

Change-Id: I5f8f70f10a0cd013a41fb7f03234bde355e5dc2c

rackerlabs/marconi,openstack/zaqar,openstack/zaqar,openstack/zaqar,openstack/zaqar