Kafka mixin should have configurable kafkaSelector for alerts

[v-zhuravlev]

Kafka mixin has hardcoded job=integrations/kafka in all alerts

Instead, it should have configurable kafkaSelector that can be injected into alerts (and dashboards too).

[v-zhuravlev]

For example:

      expr: sum(kafka_server_ReplicaManager_UnderReplicatedPartitions{job="integrations/kafka"}) by (instance) > 0