Changes for node to node communication and running workflow

[saksham219]

closes #25 and related to https://github.com/mercury-app/mercury/issues/15 closes #1

• removes docker file for mercury container from this repo. It has been moved to the container repo.
• changes /nodes endpoint response to include input and output code snippets for that node
• exposes a /nodes/<node id>/notebook endpoint for a PATCH request to either run a code snippet or ask the node to write_to-json variables from its running kernel. For request body look at updated postman docs.
• builds the input and output code snippets from the input output variables for the node also relying on the edge json(which is an edge attribute).
• adds endoint to workflows for running and stopping workflow execution
• logic for executing dag and stopping it
• accept websocket from gateway to send kernel status messages.

[saifulbkhan]

@saksham219 Resolve comments here? I will then do another quick review before we merge. Need to look at what all I will have to change on top of this.