packagesets: add basic imagetype yaml to merge pkgSets

[mvo5]

[this is my personal winner in the race for best-yaml-for-shareing-pkgsets, based on a new idea of @ondrejbudai (thanks!), see also https://github.com//pull/1294 and https://github.com//pull/1296 and https://github.com//pull/1295]

This commit adds a basic imagetype yaml to support merging of package sets This is a useful feature so that we can e.g. avoid duplicating the cloud_base or anaconda package sets.

It works by adding new toplevels "image_types" in the package_sets.yaml and then using anchors/refs to refer to them in the image_type yaml. To allow sharing yaml that is not part of an image_type it also introduces a top-level .common that can have any yaml in it.

It allows us to write:

.common:
  base: &base_pkgset
    include: [from-base-inc]
    exclude: [from-base-exc]
    condition:
      distro_name:
        test-distro:
          include: [from-base-condition-inc]
          exclude: [from-base-condition-exc]

image_types:
  other_type:
    package_sets:
      - &other_type_pkgset
        include: [from-other-type-inc]
        exclude: [from-other-type-exc]
  test_type:
    package_sets:
      - *base_pkgset
      - *other_type_pkgset
      - include: [from-type-inc]
        exclude: [from-type-exc]
        condition:
          distro_name:
            test-distro:
              include: [from-condition-inc]
              exclude: [from-condition-exc]

The downside of this approach is that:

1. it requires the anchor to be there before the ref, which can be confusing (we could solve this later via includes)
2. if the anchor is forgotten the error becomes very obscure
3. its a bit "hardcore" yaml

The upside is that it is very symetrical and actually feels quite nice.