Zero config for system tests

[ra-tolson]

The current setup process for gips system tests can be made more convenient. Presently users have to copy a config file and edit two items in it; it would be nice if this could be avoided. Implementation:

• Have system tests pass an environment variable to gips_* commands letting it know that it needs to alter its config.
• Extend the gips configuration facility (the way it chooses its settings file) such that gips automatically looks for that environment variable and alters its config if a true value is asserted.
• Establish settings (that are committed to version control, which means they aren't settings at all, but hardcoded globals) that are suitable for system tests. Specifically:
  • data repository in /tmp
  • output dir in /tmp
• Continue to support the current setup, wherein the data repo path can be configured explicitly: If pytest.ini contains a setting for it, then that's what it uses. Likewise if the environment variable isn't asserted, gips falls back to its own config, which may be specified by the user in standard ways.

[ra-tolson]

Update: I thought MODIS auth would cause a problem, but it turns out we can leave that part of config manual while setting up defaults for the rest.

For now I'm tabling this effort since I've been given other priorities.