Add support for user.xyz options in distrobuilder

[stribika]

Currently you can only pass options to distrobuilder which are defined in the reference.

It would be useful to support arbitrary options which can then be referenced in the template. For example, one could call

distrobuilder build-lxc $distro.yaml $distro --options user.output=$datadir

with a YAML file like this

targets:
  lxc:
    config:
    - type: all
      content: |-
        lxc.arch = {{ image.architecture_personality }}
        lxc.mount.entry = {{ user.output }} home/user/export none bind,create=dir

A related forum post can be found here.