Ability to change log level

[Meemaw]

By default, fake-gcs-server is quite spammy, doing info logging for every request.

Add an ability to change the log level to e.g warn/error

[fsouza]

That sounds good to me. We can add a new flag (and eventually an environment variable when we solve #561).

[RachitSharma2001]

@fsouza @Meemaw I was wondering if I could pick this task up. My idea is to include a log-level flag which can be one of two options:

1. info - log all http requests, warnings, and errors (basically everything)
2. warn-error - log only warnings and errors
   Let me know what you think of this idea and if there is anything else I should change or add.

[fsouza]

Hey @RachitSharma2001, I was thinking the flag would be a string and take anything that logrus accept. You could call ParseLevel and if that fails, you report an error to the user indicating invalid log level or something like that.

[RachitSharma2001]

That sounds good! I have made a PR for this. Let me know if there is anything that should be changed or added.