main.go: find hashring name in label, not sts name

[squat]

This commit modifies the controller to match statefulsets to hashrings using a label rather than the statefulset name. This is important because resource names are tied to infrastructure logic, not business logic. Furthermore, most thanos-receive statefulsets names will always have a thanos-receive- prefix, meaning that every hashring name would also have to have that prefix. This change allows us to decouple infrastructure and business.

cc @metalmatze @kakkoyun

[brancz]

lgtm