azure: Communication configuration

_This issue was originally opened by @rickard-von-essen in https://github.com/hashicorp/packer/issues/4174 and has been migrated to this repository. The original issue description is below._

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Related to #4172

- [ ] Don't set a default `ssh_username` [in a builder](https://github.com/hashicorp/packer/blob/676c3a21bf1858d5786ea3c74efbd1e7a2a24ad4/builder/azure/arm/config.go#L688).
- [ ] [The default to both](https://github.com/hashicorp/packer/blob/676c3a21bf1858d5786ea3c74efbd1e7a2a24ad4/builder/azure/arm/config.go#L580) `ssh`and `winrm` communicator is really weird and undocumented. I think it should be removed, even if it breaks BC, or at least clearly documented. Currently some valid configurations will be ignored or not work as expected.

Start in builder/azure/arm/config.go at the referenced lines around the ssh_username and communicator defaults, and review related issue #4172. Determine how the defaults affect valid SSH and WinRM configurations, then make the behavior explicit or documented and verify that the listed configuration cases are handled as intended.

Written by the indexing model from the issue text.