NIFI-3101: Improve Get/Modify/PutHTMLElement URL

[ijokarumawak]

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:

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

• 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.

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

• Is your initial contribution a single, squashed commit?

For code changes:

• Have you ensured that the full suite of tests is executed via mvn -Pcontrib-check clean install at the root nifi folder?
• Have you written or updated unit tests to verify your changes?

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.

• Added detailed description about how the URL property works with
  GetHTMLElement
• Added Expression support with URL
• Made URL property dynamic with ModifyHTMLElement and PutHTMLElement,
  since it won't be used to alter HTML element and need not to be
  specified. Making it a dynamic property let existing processor configuration stays valid

[ijokarumawak]

Confirmed old processor configurations work with this update by using this template created with NiFi 1.0.

I hope this change makes the purpose of the URL property clearer.