Retry the sfdisk command up to 3 times.

Updates the nova baremetal-deploy-helper so that we retry
the fdisk command up to 3 times.

Previously the command would sometimes intermittently fail with
'Device or resource busy' errors. There doesn't seem to be a good way
to check/verify this other than to retry.

Fixes LP Bug #1191084.

Change-Id: I18be73cba9c5bf5a4f9cf229dedc4494666c9b35
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: I9e6766560a8d8bf0d33059fc2547cbb8d2f861f6
Merge "Add err_msg param to baremetal_deploy_helper."
Merge "Retry the sfdisk command up to 3 times."