Support ssh_proxy_* variables for Linux images

[sestegra]

Summary of Pull Request
Add support of ssh_proxy_* variables for Linux images.

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 a something else.
  Please describe:

Context of the Pull Request*
#39

Related to Existing Issues
Issue Number: N/A

Test and Documentation Coverage

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

Breaking Changes?

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

[tenthirtyam]

@sestegra Can you confirm the following in the PR: Tests have been completed (for bug fixes/features)?

[sestegra]

As discussed in thread #39, I can't test the proxy configuration with current local HTTP server.
I did some tests with my locally updated Ubuntu 20.04 user-data (using CD instead of HTTP server).

[tenthirtyam]

Thanks for the update. I'll hold this one and see if I can test it soon. I'm working on some other changes first in my spare time.

Ryan

[tenthirtyam]

Please run a 'packer fmt' against the updated 'linux-*.pkr.hcl' files to align formatting of the variables.

Ryan

[github-actions]

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