Convert module to PDK

[malcyon]

Convert module to PDK so we can use the latest release and validation tools.

Fixes #33

Reference:

• https://puppet.com/docs/pdk/1.x/pdk_converting_modules.html
• https://puppet.com/docs/puppet/7/modules_publishing.html

[malcyon]

Files that pdk convert created that I did not include:

• .gitlab-ci.yml
• .travis.yml
• appveyor.yml

We don't use those services, so no need to have these files. Everything else PDK wanted to add, I left in.

[malcyon]

@bernd A few observations:

• The tarball created by pdk build doesn't contain checksums.json, either. It looks the same as the tarball I downloaded from Github off the tag.
• The documentation on publishing a module say to upload it via the website: https://puppet.com/docs/puppet/7/modules_publishing.html#modules_upload_to_forge
• If I run pdk help release, it shows the command as Experimental.

So it seems like the way I published 0.9.1 is fine. But at least this conversion modernizes the module.

[malcyon]

Tested by running vagrant up ubuntu2004. Graylog still comes up just fine.