Mark a node as not allocated on discovering

To avoid a race on the sledgehammer (where a node can be discovered, but
not yet marked as not allocated), do that as early as possible.

The code in the deployer barclamp to auto-allocate (if use_allocate is
set to false) will still work fine.
Merge pull request #518 from vuntz/not-allocated-on-discovering

https://github.com/crowbar/barclamp-crowbar/pull/518

Merged by devtool for cloudsleuth