* Alerting: Rename remote.ExternalAlertmanager to remote.Alertmanager
* Alerting: Send alerts to the remote Alertmanager
* add ticker to readiness check, add tests
* use options when creating a new sender.ExternaAlertmanager
* unexport defaultMaxQueueCapacity
* delete unused defaultConfig field
* add debug log line when sending alerts to the remote alertmanager
* move and refactor readiness check
* update tests to not include defaultConfig