Add e2e tests for APM / Kibana association

[barkbay]

Following-up #3043 we should add some end to end tests to validate that an APM Server is correctly associated with Kibana.

[barkbay]

I think we can implement the following scenario:

1. Create an agent configuration using the Kibana API, the API is officially documented from 7.7.0 but is available since 7.5.0

   PUT https://x.x.x.x:5601/api/apm/settings/agent-configuration

   The following HTTP body creates the default agent configuration (a.k.a. All) :

   {
       "service": {},
       "settings": {
           "transaction_sample_rate": 1,
           "capture_body": "errors",
           "transaction_max_spans": 100
       }
   }

2. Fetch the configuration from the APM Server with the Agent configuration API

   GET https://y.y.y.y:8200/config/v1/agents?service.name=All

   Compare the results:

   {
     "capture_body": "errors",
     "transaction_max_spans": "100",
     "transaction_sample_rate": "1"
   }

[pebrc]

Very good idea!