add support for --diff

[ktdreyer]

Ansible has the ability to show the "diff" for a configuration:
https://blog.networktocode.com/post/generating-diff-with-ansible/

We might be able to use this instead of rolling our own diff output. As one example, koji_tag's ensure_inheritance() prints the "before" and "after" settings using describe_inheritance().

[simonbaird]

Now ktdreyer/errata-tool-ansible#207 is merged, I'm planning to take a look at this one.

FYI there's an internal tracker for it at https://issues.redhat.com/browse/CWFCONF-1064 .

[ktdreyer]

I looked at this recently for a small module (koji_tag_packages.py).

In ktdreyer/errata-tool-ansible@720e943, we always had a single huge dict of the current state (eg. from get_product(), or get_release(), etc), and it's straightforward to diff that against the module's params.

In contrast, the modules in koji-ansible have many different "get" methods scattered throughout. It's difficult to build up a total "current state" dict to diff against.

This reminds me of #30 . I'm thinking of refactoring the koji-ansible modules to have two stages: a "gather" stage where we read all the information we're going to need from the hub (ideally in a single multicall RPC). Then the second stage would "apply" the changes the user requested.