[V2] alertmanager runs even when metrics disabled

[jshufro]

In external client mode:

[+] Running 3/3
 ✔ Container rocketpool_alertmanager  Started                                                                                                                                                                                                                                                                            0.2s 
 ✔ Container rocketpool_node          Started                                                                                                                                                                                                                                                                            0.2s 
 ✔ Container rocketpool_validator     Started