add check_elasticsearch test

[PetrDlouhy]

Add simple check for ElasticSearch.
It requires elasticsearch package to be installed to perform this test, but if the test is not active, it should not be required.

[ipmb]

Thank you for sharing this. It looks well written, but I'd prefer to keep the built-in checks minimal and limited to what's built-into Django.

[PetrDlouhy]

@ipmb OK, I created a separate project for such tests: https://github.com/PetrDlouhy/django-alive-checks
Would you be willing to add a link to it to README?
I will test it first on my project and when I tune it up I will make PR.