Register result when Agent install task is run

[KSerrania]

What does this PR do?

Registers the result of the Agent install in the datadog_agent_install variable so that it can be used in post_tasks.

Motivation

Allow custom post tasks to know if the Agent installation did install something.
Fixes #266.

[albertvaka]

Is splitting in multiple files needed? There isn't any strong reason, but I think I liked it better on single files.

In any case, LGTM.

[KSerrania]

Is splitting in multiple files needed? There isn't any strong reason, but I think I liked it better on single files.

Variable registering happens even if the job is skipped, so for example if we had:

- name: Install latest datadog-agent package
  ...
  register: datadog_agent_install
  when: datadog_agent_debian_version is not defined

- name: Install pinned datadog-agent package
  ...
  register: datadog_agent_install
  when: datadog_agent_debian_version is defined

in the same file, and installed the latest version, we'd have datadog_agent_install.changed = False and datadog_agent_install.skipped = True since the second register would override the first one.

To avoid that, I needed a way to make the unused task not be in the run list at all, which I managed to by putting them in separate files and only including the files when needed.

There's an alternate solution that we can use if you like it better: we register two internal variables (datadog_agent_install_latest and datadog_agent_install_pinned) and then create a new task which checks these two vars and sets datadog_agent_install to the one that was not skipped.

[albertvaka]

Your current solution is fine. Thanks for your explanation 🙇