Support custom username

[jpculp]

Issue number:

#31

Description of changes:

By default, the admin container's local user is ec2-user.
The first commit allows for the local user to be named something else.

The second commit cleans up the README.md by normalizing spacing of the examples.

Testing done:

• No custom user-data.
• User-data with custom user: "user-data": "bottlerocket"
• User-data with malformed data: "user-data": {]
• User-data with an empty string: "user-data": ""

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[jpculp]

• Tweaked the logic to verify the validity of the user specified in user-data after the jq.
• Added test case for user being set to an empty string.

[jpculp]

I improved the logic a little to be more condensed, but also factor in the scenario where jq returns an error that somehow passes the regex.

[zmrow]

LGTM

🕹️

[jpculp]

Made a small cosmetic change for consistency and retested.