2800: Allow api() caller to specify api host and token.
Merge branch '2800-python-global-state' into 2800-pgs

Conflicts:
	sdk/python/arvados/api.py
2800: Merge branch 'master' into 2800-pgs
2800: Clean up exceptions, variable names, handling of discoveryServiceUrl vs host args.
2800: Remove unnecessary sha1. Just use the tuple as an array key.
2800: Restore deleted comment which is still relevant.
2800: Sort import statements.
Merge branch '2800-pgs' refs #2800