problem: pulp_installer doesn't document required variables

[mikedep333]

correctly or provide clear errors if they are unset

Solution:

1. Error early if they are null, unset an empty string, or
   an empty dict with a clearer error message.
2. Remove pointless setting of some to empty values.
3. Correct the documentation: They have no default values.

fixes: #6958

Test vars:

pulp_install_plugins: {}
  # galaxy-ng: {}
  # pulp-ansible: {}
  # pulp-certguard: {}
  # pulp-container: {}
  # pulp-cookbook: {}
  # pulp-deb: {}
  # pulp-file: {}
  # pulp-gem: {}
  # pulp-maven: {}
  # pulp-npm: {}
  # pulp-python: {}
  # pulp-rpm:
  #   prereq_role: "pulp.pulp_rpm_prerequisites" # RPM needs a prereq_role: https://galaxy.ansible.com/pulp/pulp_rpm_prerequisites
pulp_settings:
  secret_key:
  content_origin: ""

[pulpbot]

Attached issue: https://pulp.plan.io/issues/6958

[mikedep333]

I'm running into some weird syntax errors. I'll resume work on this after #326

[mikedep333]

@dkliban I made a bunch of changes when I rebased and fixed this, so requesting re-review.

[bmbouter]

Does it make sense to add a .removal notice for these two vars now being required?

[mikedep333]

My test last week was that the empty string default would fail mid-play. So it's not really a removal of any functionality. I suppose that users could be comparing to the empty string in their own playbook or roles, but it seems very unlikely.

[bmbouter]

I like how helpful this error message is. 👍

[mikedep333]

FYI, it is all outputted as one line though.

I found instructions for the debug module (rather than assert) module to do multiple lines via an array, but it just becomes an array outputted as JSON, still one line.

[bmbouter]

This looks great, thanks @mikedep333 !