[murano-agent] python version should refer to "headers" attribute instead of "header" for message object

When the python version of the agent is deployed on a VM, I get the following error:

2014-05-02 22:25:58.873 3715 DEBUG amqp [-] Open OK! _open_ok /usr/local/lib/python2.7/dist-packages/amqp/connection.py:637
2014-05-02 22:25:58.877 3715 DEBUG amqp [-] using channel_id: 1 __init__ /usr/local/lib/python2.7/dist-packages/amqp/channel.py:80
2014-05-02 22:25:58.885 3715 DEBUG amqp [-] Channel open _open_ok /usr/local/lib/python2.7/dist-packages/amqp/channel.py:438
2014-05-02 22:25:58.889 3715 DEBUG amqp [-] using channel_id: 2 __init__ /usr/local/lib/python2.7/dist-packages/amqp/channel.py:80
2014-05-02 22:25:58.897 3715 DEBUG amqp [-] Channel open _open_ok /usr/local/lib/python2.7/dist-packages/amqp/channel.py:438
2014-05-02 22:25:58.917 3715 ERROR murano-common.messaging [-]
2014-05-02 22:25:58.925 3715 DEBUG amqp [-] Closed channel #2 _do_close /usr/local/lib/python2.7/dist-packages/amqp/channel.py:104
2014-05-02 22:25:58.933 3715 DEBUG amqp [-] Closed channel #1 _do_close /usr/local/lib/python2.7/dist-packages/amqp/channel.py:104
2014-05-02 22:25:58.937 3715 WARNING muranoagent.app [-] Communication error
2014-05-02 22:25:58.937 3715 TRACE muranoagent.app Traceback (most recent call last):
2014-05-02 22:25:58.937 3715 TRACE muranoagent.app File "/usr/local/lib/python2.7/dist-packages/muranoagent/app.py", line 123, in _wait_plan
2014-05-02 22:25:58.937 3715 TRACE muranoagent.app msg = subscription.get_message(timeout=5)
2014-05-02 22:25:58.937 3715 TRACE muranoagent.app File "/usr/local/lib/python2.7/dist-packages/muranoagent/common/messaging/subscription.py", line 48, in get_message
2014-05-02 22:25:58.937 3715 TRACE muranoagent.app return message.Message(self._connection, msg_handle)
2014-05-02 22:25:58.937 3715 TRACE muranoagent.app File "/usr/local/lib/python2.7/dist-packages/muranoagent/common/messaging/message.py", line 32, in __init__
2014-05-02 22:25:58.937 3715 TRACE muranoagent.app self.id = message_handle.header.get('message_id')
2014-05-02 22:25:58.937 3715 TRACE muranoagent.app AttributeError: 'Message' object has no attribute 'header'
2014-05-02 22:25:58.937 3715 TRACE muranoagent.app

The Message object is of the class kombu.transport.pyamqp.Message

The requirements.txt file for the python version of the murano-agent requires kombu 2.4.8 or greater, in which there is no "header" attribute for this Message class, but there is a "headers" (with an "s") attribute.