MSI: move msi commands to its own command module in knack style

So far, I put the az identity command tree inside azure-cli-vm command module as i want to avoid the on-going knack conversion. But after the knack release, i will do the work for the new command module. The whole thing is transparent to end users

[yugangw-msft]

yes, a new command module would be too heavy and expensive. I agree we move them to azure-cli-role. MSI essentially is a service principal as a resource. So this makes sense.

[tjprescott]

@adewaleo @yugangw-msft instead of putting them in their own module, would it make sense to put them in a module that already exists? For example azure-cli-role has to do with authentication, which is what MSI is. I’d prefer not to put them in a stand-alone module, but if there’s a more logical place for them to live besides the VM module that sounds good to me.