Allow enable/disable of Prometheus endpoints

Since these are unauthenticated, it might be the case that an
administrator wishes to completely disable these metrics endpoints.
Provide that option via pvc.conf through pvc-ansible's existing
enable_prometheus_exporters option and the new enable_prometheus
configuration flag.

Defaults to "yes" to provide all functionality unless explicitly
disabled, as the author assumes that the PVC API is secured in other
ways as well and that metric information is not completely sensitive.

daemon-common/config.py

@@ -176,6 +176,7 @@ def get_parsed_configuration(config_file):
             "enable_storage": o_subsystem.get("enable_storage", True),
             "enable_worker": o_subsystem.get("enable_worker", True),
             "enable_api": o_subsystem.get("enable_api", True),
+            "enable_prometheus": o_subsystem.get("enable_prometheus", True),
         }
         config = {**config, **config_subsystem}