Add option to specify an existing SSH key for Azure VM

[lumaxis]

Maybe people would like to specify an existing SSH key or a specific key used for Azure work instead of generating a new key each time

[gorzell]

I am not sure if it actually generates a new key every time. I think it looks in $HOME/.ssh/id_rsa* and if something exists it will use it. I forget where in the docs I saw that, but I will try and track it down.

[lumaxis]

True!

Generate SSH public and private key files if missing. The keys will be stored in the ~/.ssh directory.

From az vm create

[lumaxis]

Still, I'd like to use a separate, existing SSH key for all things Azure, so being able to specify that would be nice.