[test] Balancer relation amulet test

[nicopace]

Implement a test to ensure the balancer relation works as expected.
To do this, we should deploy apache and a second charm, and relate them via the balancer relation, and then scaling the second charm to 3.
By default, the balancer relation works as a round-robin balancer, so we could test this by doing several requests to a service that provides different values per unit.
Then we can check if the config scaled correctly, by doing subsequent request and ensuring that each of them answers as expected.