This fixes an issue where it was impossible to specify a collector that
was available but not selected by default.
Instead of checking whether chosen collectors are valid at flag parse
time, this moves the check into the builder, where we can reference it
against the availableStores in the builder. As a bonus, the error
message also prints out a list of available collectors:
```
kube-state-metrics --collectors non-existent-collector
I0618 15:23:34.517532 50719 main.go:88] Using collectors non-existent-collector
F0618 15:23:34.519132 50719 main.go:90] Error: collector non-existent-collector does not exist. Available collectors: persistentvolumeclaims,configmaps,limitranges,nodes,namespaces,persistentvolumes,pods,replicasets,services,cronjobs,deployments,ingresses,horizontalpodautoscalers,jobs,poddisruptionbudgets,secrets,certificatesigningrequests,daemonsets,endpoints,storageclasses,replicationcontrollers,resourcequotas,statefulsets
```
Since the removal of collector, this introduces both the concept of the
store and the resources instead of collectors that the user passes in.
The user facing logs and flags were not changed as that would be a
regression.
Ergin Babani