enhance: Add support for SLES 15 (SP3) machine image

[gcblack]

Summary of Pull Request

Adds support for SUSE Enterprise Linux 15 machine image.

• 15.3 / SP3 for initial introduction

Known issues:

• HTTP-based kickstart only.

Will note in the Release Notes and CHANGELOG.md

Type of Pull Request

• This is a bug fix.
• This is an enhancement or feature.
• This is a code style / formatting update.
• This is a documentation update.
• This is a refactoring update.
• This is something else.
  Please describe:

Related to Existing Issues

Closes: #199

Test and Documentation Coverage

• Tests have been completed (for bug fixes / features).
• Documentation has been added / updated (for bugfixes / features).

Breaking Changes?

• Yes, there are breaking changes.
• No, there are no breaking changes.

[tenthirtyam]

LGTM! 🚀

Great results in the initial tests. Will run a final set before merge!

[tenthirtyam]

Solid testing!

Merging!

[github-actions]

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