Fix acceptance test initialization

### Fix acceptance test initialization

Acceptance tests were failing because they weren't waiting for broker
initialization. This patch reforms ActorSystemBootstrap to use an 
initialization message and to make BrokerSupervisorActor to notify
when such initialization is done successfully.

Integration test fixture waits for the broker and the first acceptance
test can be enabled. Cheers!

### Testing

Test-driven-refactor

### Coverage

94.24% for the coinffeine-server module