Fix 180: Add environment variables to Click CLI options

[calizarr]

This will add environment variables as extra options for the index-url flag. Click will split on whitespace for multiple values by default.

Added a possibly unnecessary test to make sure that flags override environment variables.

Signed-off-by: Cesar Lizarraga [email protected]

[calizarr]

I believe I've just rebased and added my signature.

[TG1999]

@calizarr not all the commits have the sign-off, neither the PR description have it

[calizarr]

Alright, I'll try again. Not entirely sure how to add it to the PR description though

[TG1999]

Something like this

Signed-off-by: Name <email>

in the end of PR description

[calizarr]

All commits should be signed off now. @pombredanne @TG1999

[MarcelBochtler]

@pombredanne what is missing from this PR to get it merged? We also need #180 fixed, and I would like to avoid working around it in ORT.