Credentials managed with the Flow

Hi,

This is my pull request for the "save on deploy" part of #93.
The credentials are now fetched automatically when editing a node (as before) and only send when the user click the deploy button (with the flow).

I use a new property of the node : node._creds to store all the value needed. When the flow has been successfully send and save, the credentials information are removed from the node.

If the PR is good for you, I'll squash it.