Allow volumes with sizes less than 10G in Acornfile (#1232)

[thedadams]

If a volume is defined in an Acornfile with a size less than 10G, then the size would be overwritten by the parser to be 10G. This was because it mistakenly thought there was a volume binding with size 10G. After this change, volumes defined in an Acornfile with size less than 10G will not be accidentally overwritten.

Issue: #1232

Checklist

• The title of this PR would make a good line in Acorn's Release Note's Changelog
• The title of this PR ends with a link to the main issue being address in paranthesis, like: This is a title (#1216). Here's an example
• All relevant issues are referenced in the PR description. NOTE: don't use GitHub keyworkds that auto-close issues
• Commits follow contributing guidance
• Automated tests added to cover the changes. If tests couldn't be added, an explanation is provided in the Verification and Testing section
• Changes to user-facing functionality, API, CLI, and upgrade impacts are clearly called out in PR description

[jacobdonenfeld]

LGTM, small comments and questions.

One thing I couldn't find when walking the code is adequate usage of MinSize/MinSizeQuantity. Is there feedback to the user when defining or updating a volume attempting to shrink it, or when below the minimum value? Should there be a check to ensure its above the minimum size somewhere in volume.go:toPVCs? I see it referenced once.