prometheus: Let name and replicas be set in _config

Before #260, the Prometheus name and number of replicas could be
configured in `_config.prometheus.name` and
`_config.prometheus.replicas` respectively.

It isn't the case anymore, which means that configurations that did set
a custom name for Prometheus will get a second Prometheus instance
called `k8s` when they upgrade kube-prometheus.

This commit adds back the ability to configure both of these parameters
in `_config`.