feat: Parameterize os_disk_size_gb for Mariner vhds

[Camelron]

What type of PR is this? /kind feature

Parameterize os_disk_size_gb for Mariner vhds. For our Mariner Kata images, we were running out of space in the rootfs of the original VHD. By piping in this extra parameter, we can customize our kata images to have more space by default.

• [ ] includes documentation
• [ ] adds unit tests
• [ ] tested upgrade from previous version

Special notes for your reviewer:

Release note:

none

[coveralls]

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Warning: This coverage report may be inaccurate.

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• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 79.007%

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Totals
Change from base Build 6788657094:	0.0%
Covered Lines:	2243
Relevant Lines:	2839

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💛 - Coveralls

[cameronmeissner]

feel free to reopen without using a fork if needed