Manage livy endpoint from magics

[aggFTW]

This will be the API:

• %spark add session_name language conn_string
  will create a session against the endpoint specified
• %spark info
  will display the info for the sessions created in that notebook
• %spark config <configuration_overrides>
  will add session configs for subsequent sessions
• %spark info conn_string
  will list the sessions for a given livy endpoint by providing session_id, language, state
• %spark delete session_name
  will delete a session by its name from the notebook that created it
• %spark delete conn_string session_id
  will delete a session for a given endpoint by its id
• %spark cleanup
  will delete all sessions created by the notebook
• %spark cleanup conn_string
  will delete all session for the given livy endpoint

This covers #56, #75, and #76 for magics in Python kernel.
We are not designing the API for the wrapper kernels here and we'll tackle that as a separate improvement.

ping @msftristew @ellisonbg to take a look when they can

[msftristew]

What information will be included when you do %spark info?

Anyway, API looks good.

[aggFTW]

It will print the sessions you've created, listing endpoint, session_id, language, state

[aggFTW]

Due to the way that argument parsing is done for magics, the only way to allow for configuration overrides for session creation in the add case is to add another API:

• %spark config configuration_overrides

This will save the configuration overrides and every session added after it will start with that configuration.

Thus, add will become:

• %spark add session_name language conn_string