one linux VHD build mode, eliminate gen1, gen2, and sig modes for linux VHD builds

[cameronmeissner]

What this PR does / why we need it:

• Merged gen1, gen2, and sig modes into one linuxVhdMode for Linux VHD builds
• Windows VHD builds will maintain their current state and retain all three original modes
• We now build MarinerV1 Gen1 VHD images from a marketplace base image instead of a pre-supplied VHD blob
• All Linux VHD builds through packer will now output a managed image which then gets published to a SIG version

Which issue(s) this PR fixes:

Fixes #

Requirements:

• [ ] uses conventional commit messages
• [ ] includes documentation
• [ ] adds unit tests
• [ ] tested upgrade from previous version

Special notes for your reviewer:

Release note:

none

[AbelHu]

@wanqingfu could you help to review this PR to make sure that Windows SIG build and GEN2 build still work with this change?

[wanqingfu]

I've gone through all the file changes. This PR is compatible with WINDOWS SIG build and GEN2 build @AbelHu. For the windows part changes, they are basically only renaming the GEN2_CAPTURED_SIG_VERSION to CAPTURED_SIG_VERSION - functionality remains the same.

Confirmed with @cameronmeissner and will continue to follow up on this. I've been monitoring the pipeline also, and I don't think I'm seeing any fail in most recent runs relating to this PR.