fix(windows packer): install_git variable and WinRM timeout adjusted to 15m

[henrykie]

Issue number:
Closes #308

Summary

Creates install_git variable and defaults to True.

Changes

Please provide a summary of what's being changed

See summary.

User experience

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Users can disable Git installation on Windows build agent AMI with a variable.

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• I have performed a self-review of this change
• Changes have been tested
• Changes are documented

Is this a breaking change?

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[jorisdon]

The INSTALL_GIT environment variable is not actually used, so I think we should remove it instead. I've submitted a PR that does so (and fixes a few more issues): #318

[jorisdon]

Closing in favor of #318 which has an alternative fix, and has the WinRM timeout change incorporated into it as well.