[RFE] Add create-user flag to Azure-init CLI

## Current situation
Azure-init is aiming to improve its modularity by adding flags for each feature. We would like to be able to create the admin user using one of these flags.

## Ideal future situation
We are able to call one of the below flags to have azure-init pull IMDS data, get the user's information, and write it to an available backend.

## Implementation options
- `azure-init --create-user`
- `azure-init --create-admin-user`

## Additional information
This should not be an exclusive flag. We would like the ability to use multiple flags in a call to azure-init (`--create-user --configure-sshd`)