Fix multiple calls to bc-provisioner-json.rb

Up until now, bc-provisioner-json.rb was keeping leading and trailing
newlines, which was resulting in empty lines when calling
bc-provisioner-json.rb. The issue is that empty lines break crowbar in
interesting ways. We'll fix that too, but at least, we can already make
sure to not cause that bug.
Merge pull request #1853 from vuntz/fix-multiple-bc-provisioner-json.rb

Fix multiple calls to bc-provisioner-json.rb