Respect the PROXY_ENABLED environment variable

[mssola]

Description

This is a special environment variable set in SUSE systems and that is managed by tools like YAST. The usage of this environment variable is not standard and hence the net/http/httputil library does not support it.

Therefore, this commit adds an extra check for this environment variable when setting up the HTTP client.

See SUSE/connect@b2758ad

How to test it

1. Build with this patch applied.
2. Check that the environment variable PROXY_ENABLED is respected. That is, even if HTTP_PROXY has been set, setting PROXY_ENABLED to false will make SUSEConnect to ignore the setup.
3. Note that not setting it at all (e.g. leave a blank value or not setting it at all) will tell SUSEConnect to go on as usual.

[mssola]

[URAS] [connect-ng] Make sure that the PROXY_ENABLED environment variable is respected

[skazi0]

lgtm

[mssola]

@skazi0 @djoreilly note that I have basically reverted to the original code from this PR. That is, we don't use ParseBool and instead just do some basic comparisons. I've done this because ParseBool is not being too convenient on this case. Notice that the /etc/sysconfig/proxy files use a yesno notation, so this falls out of the ParseBool scope. I have also tried to use it eitherway as a final check, but the code was becoming more cumbersome than really needed. Thus, I have kept it simple and resorted to a much simpler implementation.

[skazi0]

lgtm

[djoreilly]

IMO something like this would be easier to understand

       if strings.ToLower(strings.TrimSpace(os.Getenv("PROXY_ENABLED"))) == "no" {
               return nil, nil
       }

[mssola]

Note that it can also be "n". And users might have mistakenly set "false" or "f", and I don't think is unreasonable to support it (plus, in SUSEConnect ruby we also allow all this as you can see on the referenced commit). With this in mind, the comparison is not that easy and putting it in as a separate function reads well while allowing to hide how exactly we determine all this from the caller perspective.