Restore configuraiton options of NbExchange

[ykazakov]

It seems that the following documented configuration options are not actually used in the code:

c.NbExchange.base_url = /services/exchange
c.NbExchange.base_storage_location = /var/data/exchange/storage
c.NbExchange.db_url = mysql://username:password@my.msql.server.host:3306/db_name

Instead, the values of the following environment variables are used:

• JUPYTERHUB_SERVICE_PREFIX
• NBEX_BASE_STORE
• NBEX_DB_URL

nbexchange/nbexchange/app.py

Lines 65 to 66 in 985796a

	base_url = os.environ.get("JUPYTERHUB_SERVICE_PREFIX", "/services/nbexchange/")
	base_storage_location = os.environ.get("NBEX_BASE_STORE", "/tmp/courses")

nbexchange/nbexchange/database.py

Line 18 in 985796a

engine = create_engine(os.environ.get("NBEX_DB_URL", "sqlite:///:memory:"))

It would be nice to update documentation regarding these options.

Personally, I would prefer to use the configuration options over environment variables since this approach is more flexible: it is always possible to set the configuration options to the value of the variables, e.g.:

c.NbExchange.base_url = os.environ["JUPYTERHUB_SERVICE_PREFIX"]

[ykazakov]

I think this is also related to #26.