Allow using a custom username on newly created instances

Overview

This adds a input to server create that defines an additional username to create on the instance.

The ansible scripts have been rewritten to conditionally use the custom username when provided and the cloud-config template has been extended to dynamically populate the users.

It defaults to anything before an @ in your logged in username, which at worst will be blank. If not specified, only exouser is created.

Fixes #354

I'm sorry, but since this messes with ansible scripts too, it's stacked on top of #896 (closed)

How to Test

1. Set instanceConfigMgtRepoUrl: "https://gitlab.com/kageurufu/exosphere.git", instanceConfigMgtRepoCheckout: "custom-username" in config.js
2. Deploy a server using this branch, filling in a custom username

Guacamole should use your custom user, ssh to both exouser and your username should work with the same passphrase as well as a selected ssh key

Screenshots