How to specify internal disease normalizer environment?

[jsstevenson]

Drug indication data requires a disease normalizer instance, so the CLI currently tries to identify a disease_concepts table location and update it if necessary. However, there are a few condition branches where various values aren't properly set, raising UnboundLocalErrors.

Right now we can use DISEASE_NORM_DB_URL to set a URL, and I think if you define it as a blank string it will push up to prod (and then use the environment set in DISEASE_NORM_ENV). You can also set THERAPY_NORM_PROD to prevent attempts to check/update the disease database. I think we just need to ensure documentation is updated (if only for my sake).

Oh, also, the CLI _check_disease_normalizer method needs to check against ChEMBL too, not just HemOnc.

[jsstevenson]

Some progress in #378, still cooking