refactor `src/gardenlinux/oci`

[yeoldegrove]

What would you like to be added:

Like the other code in src/gardenlinux, oci should make better use of the GlociRegistry class.

Also think about adding a class for Cname:

class CName(object):
  _flavor = None
  _arch = None
  _version = None
  _commit_id = None

  def __init__(self, cname):
    ..parsing..

  @property
  def flavor(self):
    return self._flavor
  ...

Why is this needed:

Leander Code.