Add anti-affinity to altermanager

I noticed this in a kubescore report:

   [WARNING] StatefulSet has host PodAntiAffinity
        · StatefulSet does not have a host podAntiAffinity set
            It's recommended to set a podAntiAffinity that stops multiple pods
            from a statefulset from being scheduled on the same node. This
            increases availability in case the node becomes unavailable.

/cc @alinhg @mihai.zaharia