vm/vmss: remove the length requirement of admin-username

[yugangw-msft]

Reported by someone whose login name is 5 characters long, and could not create a VM.
Turned out shorter names are accepted by both portal and vm itself. And more important we do not have justification for why chose 6 characters long; hence it appears a bit random

[codecov-io]

Codecov Report

Merging #2400 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master    #2400   +/-   ##
=======================================
  Coverage   72.33%   72.33%           
=======================================
  Files         323      323           
  Lines       18273    18273           
  Branches     2701     2701           
=======================================
  Hits        13217    13217           
  Misses       4223     4223           
  Partials      833      833

Impacted Files	Coverage Δ
...cli-vm/azure/cli/command_modules/vm/_validators.py	73.43% <0%> (ø)	✅

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[tjprescott]

It isn't random--it depends on the underlying OS of the image.

[yugangw-msft]

It isn't random--it depends on the underlying OS of the image.

@tjprescott, could you elaborate?I also tried windows os in portal and I am able to use shorter username.
Either way, we need more details

[tjprescott]

#1943 is the issue that originally reported the issue.
This is a related issue: #582

If we remove this, we would essentially be reopening #1943. There's no perfect solution--either we choose a reasonable requirement that works across images (even if it is more conservative) or we just allow people to try and fail if the underlying image doesn't allow it.

[yugangw-msft]

@tjprescott, I prefer we just allow people to try and fail if the underlying image doesn't allow it., but let me ask portal team for their validation logic so to get a bit more data. What we have now is a bit confusing, I saw people complaining about it in #1943 as well.

[tjprescott]

We should discuss at scrum and decide the way forward for this PR.

[troydai]

@tjprescott @yugangw-msft did we reach an agreement on this? It occurs to me what is blocking this PR is about the design rather than the code. If we need more time to go through the design decision, I suggest closing this pull request for now.

[tjprescott]

We haven't brought it up in scrum yet (unless it was today's).

[troydai]

@yugangw-msft is digging deeper into this topic. I'll close the pull request for now since there is nothing to review.

[yugangw-msft]

Okay, Portal team is providing me the user name validation code. Once I get the complete info, I shall update the PR