Rally checks for internet connection even when it's not needed

[j-bennet]

Rally version (get with esrally --version): 2.5.0.dev0

Description of the problem including expected versus actual behavior:

Rally unnecessarily checking Internet connection for commands such as:

• list [races|other things]
• compare --baseline x --contender y
• generate [charts]
• possibly others?

Steps to reproduce:

Simulate being offline by providing an invalid proxy URL. Invoke a command that should work offline:

http_proxy=http://invalid esrally list races

Observe a warning:

[WARNING] No Internet connection detected.

This check should only apply to commands that need internet connection.