Allow the provisioner to provide its own sledgehammer.

This allows for CB1 and CB2 to use different versions of Sledgehammer.
Merge pull request #1979 from VictorLowther/pull-req-master-1ec7113e8c740a01d2d6b4b05f18567143c353b1

https://github.com/crowbar/crowbar/pull/1979

Merged by devtool for cloudsleuth