Fix using http2 between TO and TM when setting proxy

[mhoppa]

What does this PR (Pull Request) do?

• This PR fixes TP: internal server error on dashboard page #3902

If TM has an https proxy set that is used by TO to connect it will fail as golang uses http2 be default in that case. This PR overrides that behavior based on the net godocs https://golang.org/src/net/http/doc.go.

There is not documentation changes as it is a bug fix and there is no tests as we dont have TM set up for our API tests

Which Traffic Control components are affected by this PR?

• Traffic Ops

What is the best way to verify this PR?

Setup TM proxy with HTTPs ensure endpoint cdns/capacity work

If this is a bug fix, what versions of Traffic Control are affected?

• master (5b018e2)

The following criteria are ALL met by this PR

• This PR does not include tests OR I have explained why tests are unnecessary
• This PR includes documentation OR I have explained why documentation is unnecessary
• This PR includes an update to CHANGELOG.md OR such an update is not necessary
• This PR includes any and all required license headers
• This PR ensures that database migration sequence is correct OR this PR does not include a database migration
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)

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Test PASSed.

[mitchell852]

is there not a good way to do this globally? i'm just worried that other PRs will sneak thru without this type of change applied and generate a lot of bugs. i.e. #2305

[mhoppa]

Agreed with you @mitchell852. One question is what services besides TM/TR does the TO talk to? If we put getting http clients for each of them into a common method we can enforce it a little better as https://github.com/apache/trafficcontrol/pull/2305/files#diff-6e647a5b8d470f399423ff712efb0050R40 does.

As we talked we can set it globally per https://golang.org/src/net/http/doc.go via GODEBUG=http2client=0 # disable HTTP/2 client support but I think we have agreed that feels a little cumbersome.

[ocket8888]

It talks to TV and TS and a mail server (sort of and sometimes)

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Test FAILed.

[mhoppa]

retest this please

[ocket8888]

Compiles, works, fixes the issue.

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Test PASSed.