Fix up node role creation in the UI.

When creating new noderole bindings in the web UI, we were not proerly
building the noderole graph. This fixes up the create function in the
node roles controller to call the role's add_to_node_in_snapshot
function to properly handle creating the right parent/child
relationships, and ti fixes up some logical bugs in how we were creating
the bindings along the way.
Don't mask stage_run exceptions.

Doing that was causing more problems than it was hiding for now.
Merge pull request #745 from VictorLowther/pull-req-master-813ddc976a035cc6cb8e2f84507fac132aba7c83

https://github.com/crowbar/barclamp-crowbar/pull/745

Merged by devtool for cloudedge