Allow the use of virtual plugin in store directive

[zachomedia]

This PR adds the ability to use the virtual plugin (https://github.com/vmware/kube-fluentd-operator#reusing-output-plugin-definitions-since-v160) in the store directive as part of the copy type.

This allows for:

kube-system:

<plugin default>
  ...
</plugin>

other namespaces:

<match **>
  @type copy
  <store>
     @type default
  </store>
  <store>
    # some other store
    ... 
  </store>
</match>

[jvassev]

Can you also add a test case? I recently had a bug 10 lines below in this file. It should be trivial to add a new test for the nested

[zachomedia]

@jvassev I've added a test for this, I based it off of one of the existing tests.

[vmwclabot]

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• Commit 68ce2e4 must be signed by zachary@zacharyseguin.ca

[jvassev]

Thank you for your PR. This seems incredibly useful - I'll try to cut a release soon.