Fix syncing cache behind HTTP proxy

[AlexanderEkdahl]

This mimics the default RoundTripper and it uses http.ProxyFromEnvironment as directed by the $HTTP_PROXY and $NO_PROXY (or $http_proxy and $no_proxy) environment variables. This allows the Transport to work behind a proxy.

See https://golang.org/pkg/net/http/#Transport and https://golang.org/pkg/net/http/#RoundTripper for more details.

I assume registering custom protocol is due to internal requirements as it is not necessary when downloading files from GitHub.

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[kubeflow-bot]

This change is 

[k8s-ci-robot]

Hi @AlexanderEkdahl. Thanks for your PR.

I'm waiting for a kubeflow member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[AlexanderEkdahl]

@googlebot I signed it!

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[kkasravi]

/ok-to-test

[kkasravi]

is there a way to provide a test for this?

[AlexanderEkdahl]

This can certainly be tested and the behavior is tested in detail here: https://github.com/golang/net/blob/master/http/httpproxy/proxy_test.go. However, I don't think it is valuable to test this in kfctl as it introduces undue maintenance burden on the kfctl developers.

Using http.ProxyFromEnvironment is the default behavior for all HTTP request in Go(link) and this PR simply ensures the same behavior as the transport happens to be overridden in kfctl.

[linkvt]

Hi @kkasravi , could you please check again if this PR can be merged?
Proxy support is a really important feature for everyone behind a corporate proxy as this requires currently workarounds that almost no other tool needs, as practically everything supports proxies - and this change seems really minor.
Thanks

[Jeffwan]

/retest

[k8s-ci-robot]

@AlexanderEkdahl: The following test failed, say /retest to rerun all failed tests:

Test name	Commit	Details	Rerun command
kubeflow-kfctl-presubmit	177ca0d	link	/test kubeflow-kfctl-presubmit

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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[AlexanderEkdahl]

@Jeffwan I can't make sense of the apparent test failure. From what I can tell all of the tests passed?

[PatrickXYS]

@AlexanderEkdahl

Can you rebase and run tests?

[PatrickXYS]

/test ?

[aws-kf-ci-bot]

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@PatrickXYS: No presubmit jobs available for kubeflow/kfctl@master

In response to this:

/test ?

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[Jeffwan]

/lgtm
/approve

Thanks for the contribution

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: AlexanderEkdahl, Jeffwan

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