Provide apm-server status

[formgeist]

Having the APM Server installed and running is critical to the APM experience, which is why in terms of both onboarding and continued status, it would great to have a status provided, perhaps into Elasticsearch which can be queried, that it's setup and it's running.

[ruflin]

I suggest to start with the definition on what means apm-server is available. Also as there can be more then one apm-server how will this play into this? In case we have different definitions for available like red, yellow, green, what is the criteria for each?

This ties also into elastic/beats#3422 and should be coordinated with beats.