Code review comment for ~raharper/curtin:feature/enable-storage-vmtest-on-centos

I didn't get through the whole thing yet. Only as far as my comments stop.
will review more late.r

Feels like we need a 'distro' module.

def distro.read_distro(target=None):
   """Find the distro in target. return just distro name for now."""

also there woudl be

CENTOS='centos'
DEBIAN='debian'.

then you can avoid copying 'debian' string everywhere.