az lab vm create ignores --admin-username option

**Describe the bug**
`--admin-username` option is ignored when creating a Windows 2016 VM from a formula. If the formula specifies a default username, then the default is used. Otherwise, you get a cryptic error message.

**To Reproduce**
Without a default set:
```
$ az lab vm create -g $group --lab-name $lab --name my-vm --formula $formula --saved-secret VmPassword --admin-username MyUser
Deployment failed. Correlation ID: . A fatal error has occurred (reference Id ).
```
Using `--verbose` or `--debug` doesn't shed any light on why this command is failing. With a default username, the command works, but the specified name is ignored and the default one is used instead.

**Expected behavior**
`--admin-username` should set the admin username, overriding the formula default, if set.

**Environment summary**
homebrew azure-cli v2.0.51

P.S. I ran into four critical bugs in the `az lab vm` command (#7903 is another one that was already reported) in just a few hours of playing with DevTest Labs CLI for the first time. Does anyone actually test this stuff before it gets released?!?