This adds a gate which loops over all phases in all sites,
and performs an airshipctl apply --dry-run on them to ensure YAML
validity and schema adherence. Aside from installation tasks,
the gate is run via a makefile entrypoint so that it can be
easily consumed by developers or by non-zuul CICD platforms.
Change-Id: Ie4ab246848a580ab20c3153af1e3749a27e3f770