Enable usage of custom list of plugins when using static config file

[enothen]

This PR enables usage of a custom list of plugins when using a plugins config file, as reported on #916
Fixes #916

[enothen]

Hello reviewers,
Not sure how to handle the failed validation, as it was already in master before this PR:

$ git status
On branch master
Your branch is up to date with 'origin/master'.

nothing to commit, working tree clean
$ git log --oneline |head -5
138a977 [blacksmith] Bump version to 12.0.3-rc0
7f384ce Merge pull request #914 from wyardley/wyardley/release_12_0_2
79949c2 Release 12.0.2
3f811e5 Merge pull request #912 from kvisle/bugfix/plugin_resource_broken_310
3436f22 Make rabbitmq_plugin resource functional on RabbitMQ 3.10.x
$ bundle exec rake lint
manifests/config.pp:131:manifest_whitespace_opening_brace_before:ERROR:there should be a single space before an opening brace

Since the problem is unrelated to this PR, should I fix it here or open a separate issue?

[wyardley]

Hi. Yes, can you make a separate PR with just that fix? It passed on the last run I can find.

[enothen]

Hi. Yes, can you make a separate PR with just that fix? It passed on the last run I can find.

Done, #918

[enothen]

Can someone please rerun the tests? They should be fone now that #918 is merged.

[kenyon]

Rebasing on current master will rerun the tests.

[enothen]

Rebasing on current master will rerun the tests.

Thanks. Rebased, fixed a syntax warning in the test case, squashed and pushed again. Now I'm getting a failed job just on Archlinux, but seems to be unrelated to the changes. I'm not so sure what to make out of the logs yet.

[wyardley]

Yeah, arch failures have been there a while and are unrelated.