[Compute] Use update/PATCH instead of create_or_update/PUT in vmss/sig update command

**Is your feature request related to a problem? Please describe.**

Today, vmss update is calling into create_or_update/PUT API. This cause issues that service side validation might fail when some field not updating but still passed in PUT request. Ideally CLI should use PATCH verb. To do it, we need
- service team confirming that PATCH could be used to do all updating operation in VMSS
- full regression of vmss updating scenarios.

This will be a big effort and change fundamental of vmss command. While we've got one issue reported regarding this: vmss updating when shared image gallery in another subs will fail even no change on disk image.

It also applies to other commands like `sig`.

**Describe the solution you'd like**

**Describe alternatives you've considered**

**Additional context**