try to improve integration tests setup/teardown

[rmuir]

Integration test logic starts up ES with all defaults (and for plugins installs a plugin). the teardown phase basically does 'killall elasticsearch'. Currently this stuff is just done the same way the python release script did things.

Ideally these could be more fine-grained, maybe read from a PID file instead, maybe use a non-default port number, etc.

[spinscale]

I think we can we close, since stop-external-cluster uses the pid file for killing the process, see https://github.com/elastic/elasticsearch/blob/master/dev-tools/src/main/resources/ant/integration-tests.xml#L281

we still might work on the TODO though

[clintongormley]

@brwe please could you look at this TODO

[brwe]

todo has been taken care of in #12961. can we close this?

[clintongormley]

Looks like it, thanks @brwe