Change default server, fix bug writing config to file

[PGijsbers]

Reference Issue

None, recent issues with proxy.

What does this PR implement/fix? Explain your changes.

Sets the default url to use the api subdomain, which avoids a layer of indirection and subsequently should be stable.
It also fixes an issue with the CLI tool so that it's possible again to set variables through it.

How should this PR be tested?

Affects all endpoints. Not really that suitable for manual testing.

Any other comments?

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 84.35%. Comparing base (00d1766) to head (609f1f2).

Files with missing lines	Patch %	Lines
openml/config.py	40.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1393      +/-   ##
===========================================
+ Coverage    84.33%   84.35%   +0.01%     
===========================================
  Files           38       38              
  Lines         5325     5325              
===========================================
+ Hits          4491     4492       +1     
+ Misses         834      833       -1     

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[LennartPurucker]

This looks correct to me and the code works with "https://api.openml.org/api/v1/xml" for me bug free.

I cannot see yet how it affects the CLI

[PGijsbers]

Sorry for not being clear about the CLI fix. That's this bit of code.. it's used only by the CLI (I think). I understand that's not particularly clear since the other edit is in the cli.py file which you might reasonably expect to also have the fix that was mentioned.

Basically, if f!=field then it would write the name of the variable to itself (e.g., apikey = apikey), instead of fetching the corresponding value from the configuration and re-writing that.