Add quotas code for ELB limits

[sean-zou]

Before submitting pull requests, please see the Development documentation and specifically the Pull Request Guidelines.

IMPORTANT: Please take note of the below checklist, especially the first three items.

Summary

Add a summary of what your PR does here. This could be as simple as "adds support for X service" or "fixes default limit for Y", or a longer explanation for less straightforward changes.

Pull Request Checklist

• [x] All pull requests should be against the develop branch, not master.
• [x] All pull requests must include the Contributor License Agreement (see below).
• [x] Whether or not your PR includes unit tests:
  • [x] Please make sure you have run the exact code contained in the PR locally, and it functions as desired.
  • [x] Please make sure the TravisCI build passes or, if not, you've corrected any obvious problems identified by the tests.
• [x] Code should conform to the Development Guidelines:
  • [x] pep8 compliant with some exceptions (see pytest.ini)
  • [x] 100% test coverage with pytest (with valid tests). If you have difficulty writing tests for the code, that's fine, just mention that in the summary and either ask for assistance, or clarify that you'd like someone else to handle the tests. PRs that include complete test coverage will usually be merged faster.
  • [x] Complete, correctly-formatted documentation for all classes, functions and methods.
  • [x] documentation has been rebuilt with tox -e docs
  • [x] Connections to the AWS services should only be made by the class's connect() and connect_resource() methods, inherited from awslimitchecker.connectable.Connectable
  • [x] All modules should have (and use) module-level loggers.
  • [x] Commit messages should be meaningful, and reference the Issue number if you're working on a GitHub issue (i.e. "issue #x - "). Please refrain from using the "fixes #x" notation unless you are sure that the the issue is fixed in that commit.
  • [x] Git history is fully intact; please do not squash or rewrite history.

Contributor License Agreement

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• The contribution included in this request (and any subsequent revisions or versions of it) is being made under the same license as the awslimitchecker project (the Affero GPL v3, or any subsequent version of that license if adopted by awslimitchecker).
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