Fix --proposed in kpack-from-image for arch other than i386 and amd64.
kpack-from-image always use archive.ubuntu.com for the ubuntu mirror
instead of ports.ubuntu.com for arch other than intel.
The change here does 3 things:
a.) change variable names in maas-cloudimg2ephemeral to be consistent
with those used in kpack-from-image
a.) read UBUNTU_MIRROR and UBUNTU_MIRROR_PORTS from environment in both
kpack-from-image and maas-cloudimg2ephemeral.
c.) pass UBUNTU_MIRROR and UBUNTU_MIRROR_PORTS through sudo call
in kpack-from-image.