Add master-detail Publications

[daviddavis]

fixes #4678
https://pulp.plan.io/issues/4678

Please be sure you have read our documentation on creating PRs:
https://docs.pulpproject.org/en/3.0/nightly/contributing/pull-request-walkthrough.html

[nixocio]

This change will break tests in all other plugins as well.

[nixocio]

I will check pulp_file, and pulp_rpm.

[nixocio]

I verified: pulp_docker, pulp_file and pulp_rpm. None of them use this PUBLICATIONS_PATH variable.

[nixocio]

Error during the installation process:

TASK [Check for podman] ********************************************************
fatal: [localhost]: FAILED! => changed=true 
  cmd:
  - which
  - podman
  delta: '0:00:00.002759'
  end: '2019-04-24 15:07:51.187667'
  msg: non-zero return code
  rc: 1
  start: '2019-04-24 15:07:51.184908'
  stderr: ''
  stderr_lines: []
  stdout: ''
  stdout_lines: <omitted>
...ignoring

[codecov]

Codecov Report

Merging #94 into master will decrease coverage by 0.14%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #94      +/-   ##
==========================================
- Coverage   67.97%   67.82%   -0.15%     
==========================================
  Files          65       65              
  Lines        3016     3018       +2     
==========================================
- Hits         2050     2047       -3     
- Misses        966      971       +5

Impacted Files	Coverage Δ
pulpcore/app/serializers/publication.py	100% <100%> (ø)	⬆️
pulpcore/app/models/publication.py	70.66% <100%> (+0.39%)	⬆️
pulpcore/app/models/task.py	63.55% <0%> (-4.68%)	⬇️

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