Remove duplicate username across global config and Azure YAML

[chester-leung]

Currently, the user has to specify a username for MC², and a second username for authentication for launched Azure VMs.

We should consolidate this and have the user only input one username to simplify the configuration process - changes will have to made in the various places in the codebase that load in the config ssh_user field. Instead of loading in the Azure config ssh_user field, we should load in the global config user field.