Add support for linux VMs

[mayeut]

Resolves #32

I'm new to packer, go & provisioning in general so things might be ill formed but the example works.

Used https://ubuntu.com/server/docs/install/autoinstall-quickstart as a starting point for auto install.

A few notes:
It assumes the install process will shutdown the VM rather than restart it at the end of installation: the sample ubuntu autoinstall will just loop into the installer if the system is restarted rather than shutdown.
The auto-resize in the guest OS is probably less generic than the macOS one so a resize_disk_command was introduced. fixing the disk size is left to the user with generic shell provisioning steps.
The boot_command is hijacked to be used for the VM creation/installation rather than the following run.

[fkorotkov]

Thank you for the PoC! I think we should leave resizing of the disk out of scope for this PR. There is some very specific logic for macOS, I think in a follow up PR we can create similar steps for Linux and depending on imageConfig["os"] add different steps.

[fkorotkov]

What's this password? Should it be ubuntu?

[mayeut]

Should it be ubuntu?

Well it is:

echo 'ubuntu' |openssl passwd -6 -stdin -salt exDY1mhS4KUYCE/2
$6$exDY1mhS4KUYCE/2$zmn9ToZwTKLhCw.b4/b.ZRTIZM30JZ4QrOQ2aOXJ8yk96xpcCof0kxKwuX1kqLG/ygbJ1f8wxED22bTL4F46P0

[mayeut]

I think we should leave resizing of the disk out of scope for this PR

Ok, shall I remove everything or shall I leave the OS detection and the following ?

if isLinux {
	ui.Error("Automatic partition resizing not implemented, guest OS might not see the full disk capacity.")
	return multistep.ActionContinue
 }

The original step didn't have a check on ExitStatus so it will just print a diskutil: command not found message but it won't fail the build.

[edigaryev]

Ok, shall I remove everything or shall I leave the OS detection and the following ?

It seems that we can offload most of the OS-specific logic from stepResize to a user-managed .hcl (which knows the OS by design) and have no OS detection logic at all.

But this is for a separate PR, as noted by @fkorotkov.

[mayeut]

When rebasing on main, I had to leave a check to avoid failure with the disk parsing introduced on macOS.
The check changed to a $(uname -s) == Darwin check.
I removed the specific resize_disk_command.