NIFI-5120 AbstractListenEventProcessor supports expression language

[sbouchex]

Thank you for submitting a contribution to Apache NiFi.

In order to streamline the review of the contribution we ask you
to ensure the following steps have been taken:

For all changes:

• [ X] Is there a JIRA ticket associated with this PR? Is it referenced
  in the commit message?

• [ X] Does your PR title start with NIFI-XXXX where XXXX is the JIRA number you are trying to resolve? Pay particular attention to the hyphen "-" character.

• [X ] Has your PR been rebased against the latest commit within the target branch (typically master)?

• [X ] Is your initial contribution a single, squashed commit?

For code changes:

• [X ] Have you ensured that the full suite of tests is executed via mvn -Pcontrib-check clean install at the root nifi folder?
• [ X] Have you written or updated unit tests to verify your changes?
• [X ] If adding new dependencies to the code, are these dependencies licensed in a way that is compatible for inclusion under ASF 2.0?
• [X ] If applicable, have you updated the LICENSE file, including the main LICENSE file under nifi-assembly?
• [X ] If applicable, have you updated the NOTICE file, including the main NOTICE file found under nifi-assembly?
• [X ] If adding new Properties, have you added .displayName in addition to .name (programmatic access) for each of the new properties?

For documentation related changes:

• [ X] Have you ensured that format looks appropriate for the output in which it is rendered?

Note:

Please ensure that once the PR is submitted, you check travis-ci for build issues and submit an update to your PR as soon as possible.

[pvillard31]

Hey @sbouchex - change looks good to me, could you take care of the checkstyle issue? Just to be sure we have at least one full Travis build successful before merging. Thanks!

[sbouchex]

@pvillard31 I messed up a bity my branch, If needed I can redo it.