Add system test for libbeat internal x-pack monitoring

[ycombinator]

libbeat has code for shipping x-pack monitoring data to the elasticsearch cluster configured either via outputs.elasticsearch.* or xpack.monitoring.elasticsearch.*. This cluster then exports the given data to a monitoring cluster, which may be a separate dedicated monitoring cluster or the same cluster that received the data from the beat.

Currently there are no system tests to test this x-pack monitoring feature in libbeat. This PR creates such a system test. The test:

1. configures the test beat (mockbeat) to ship data to a cluster defined via
   xpack.monitoring.elasticsearch.hosts. Let's call this cluster ES1.
2. sets up a dedicated monitoring cluster. Let's call this cluster ES2.
3. defines an HTTP exporter in ES1's cluster settings to export monitoring data it receives to ES2.
4. enables monitoring collection in ES1.
5. starts up mockbeat and waits till it has shipped some monitoring data to ES1.
6. queries ES2 and asserts that the expected data was indexed into .monitoring-beats-* indices.

[ycombinator]

jenkins, test this

[ycombinator]

Not sure why the CLA check isn't passing, but if you're okay with that 😄 [EDIT: CLA check has passed now], this PR is ready for your review, @urso and @ruflin. Thank you!

Please note that this PR just adds a test for existing functionality. I plan to then build upon this test for the functionality being introduced in #9260.

[ruflin]

LGTM. Comment can be addressed later.