Further changes to make ES 5.0 work

[nubunto]

• Removed the ES_HEAP_SIZE env var entirely, since ES 5.0 shuts it all
  down if it finds it.
• Removed the args that are passed to ES, since he doesn't appear to
  accept them anymore. Apparently they have to be given via
  elasticsearch.yml.
• Updated the way ES_JAVA_OPTS environment variable is set, as opposed
  to ES_HEAP_SIZE. The format is -Xms[MEM][m|g] -Xmx[MEM][m|g].

[containersol]

Can one of the admins verify this patch?

[nubunto]

To see the state of these changes, see #594

[frankscholten]

@nubunto After merge conflicts have been resolved, is this one WIP or ready to be merged?

[nubunto]

@frankscholten I believe further polishing might be needed, e.g. I hardcoded megabytes as the ram size, we should improve this.
Also, we need to document a way to pass arguments as we did before. Probably piping a elasticsearch.yml file.
After that, then we should be good, I think.

[frankscholten]

Ok. Can you fix the above conflicts?

[mangolas]

What is the current situation on running ES5 from Mesos Elasticsearch, should it work after this PR is in?

[ror6ax]

Hi guys, what is the status? Will you agree to merge when conflicts are resolved?

[sadok-f]

Any update about this PR?

[ror6ax]

@containersol @nubunto - guys, can we please get the update on this?

[nubunto]

Hey guys.
I have stepped away from this for a long time. This was actually a work I needed done a year ago (sort of) but we ended up staying away from ES 5 at the time.

I am really far away from this at the moment, and I really don't have the time to maintain this PR or any work that derives from it.
I would recommend that people took a look at this branch (and the last commit I made) and wage through the issues with a new PR.