Code review comment for lp:~ltrager/maas-images/bootloaders

The bootloader import process now iterates through updates-<release>, security-<release>, and <release> to find the latest version of the package. This ensures we will always be publishing the latest version that is in the Ubuntu archives.

I also added the ability for an individual bootloader to override the global release. This allows us to pull bootloaders from different releases. Right now they all pull from Xenial.