Add err_msg param to baremetal_deploy_helper.

Updates the baremetal-deploy-helper so that we look for an
option 'e' POST parameter which contains an optional error
message string from the agent.

This works with the associated DIB branch here:
  http://review.openstack.org/#/c/33341/

And is also fully backwards compatible with existing agent scripts
which do not send the 'e' parameter.

Change-Id: I9a5914e719c2db6d2ae6d002cd386a0e48fda18d
Merge "Add err_msg param to baremetal_deploy_helper."