Update Gentoo Latest template for Stage3 date 20170504

[courtney-miles]

Updates the template for gentoo_latest to work with the latest Stage3, dated 20170504.

Changes include:

• Corrections for allow SSHD from the boot medium.
• Install kernel 4.9.16
• Corrections for install Grub
• There's also also some additional, optional, logic to make repeated attempts to get a successful automated build faster.

In regards to this last point, Gentoo requires 3 big files:

1. The bootable ISO
2. A stage3 tar file
3. The portage tree

Even though they are each only a few hundred MB, I find these all to be very slow to download (upwards of 1 hour each.) Veewee saves some time by pre-downloading the ISO, but the Stage3 and Portage tree still adds a lot of time.

I'm not a Ruby programmer, so I hobbled together a strategy where I manuall download Stage3 and Portage and package them in a single TAR and then use a veewee hook to transfer to the virtual machine. The script will extract and use these files if they exists.

I'm sure there would be a smarter strategy to achieve this, but I don't have the background in Ruby to see how. I'm open to suggestions if there is concerns about accepting this PR with this logic. Or I can remove it and save it for myself.

[FunTimeCoding]

I approve. Should I just merge PRs like this on my own or wait for your opinion, @mpapis?

[mpapis]

@FunTimeCoding if it looks good just merge :)