charms/node-app: make 'stop' idempotent

See upstream pull request:
  https://github.com/charms/node-app/pull/1
imports: refactor/clean-up for deployment

As we are getting closer to deploying/charming, let's clean up the
code base a bit.
import: add top-level import script

The idea is to have a single top-level driver script that can import
all the data types in one go.

However, to keep flexibility to allow for individual imports, we also
move common code into lib.py, so they can still be called standalone.
import/bus: update to reflect new source tree
import/bike: index on location only

Drop the index on the city/id location, as it's really not necessary.

This makes the bike import similar to the other transport modes.
import/bike: name of collection must be plural

Matching this convention makes working with nodejs easier.
import/metro: update to reflect new source tree
import/train: update to reflect new source tree
import: factor out common drop_and_recreate()

DRY, Yo.
import: pluralize collections, normalize API

In prep for nodejs/mongoose conventions, take the time now to pluralize the
collection names.

Make each module's import function a standard name so we can dynamically
call it from import.py.
import: pluralize all the things

Finish the great pluralization project.
import: make console outpt prettier