Add common pipeline variable combinations

[ehelms]

I often find myself running these combinations and storing those locally when running pipelines.

ansible-playbook pipelines/install_pipeline.yml -e @pipelines/vars/common/katello_nightly_centos7.yml

[evgeni]

probably worth documenting in https://github.com/theforeman/forklift/blob/master/docs/testing.md

edit: but the whole testing section could use some updates 🙈

[ekohl]

Perhaps time to update #1064 so we have a nice CLI tool instead?

[ehelms]

Yea, I'll come around to clean up and documentation after #1239 . For now, this makes my life easier. Thanks!