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: I9c0733432dee5d556d6d2460db42f844e277c8de
Merge "Retry the sfdisk command up to 3 times."