Generally a data collector can be deactivated in the config and each
data collector defines own conditions to show/hide it.

A hidden data collector is not shown in /1/list/collectors,
/1/report/all. The relevant config is requested from the RConfD.

   * Added new constant dataCollectorNames: The consistency is checked
     at compile time thanks to template haskell.
   * cfgupgrade: add datacollectors section in cluster. The section
     currently has a single entry: 'active'
   * Added Arbitrary instances for the relevant types
   * Move Ganeti.Monitoring.{Server,Types}.DataCollector
   * Move {Monitoring.Server,DataCollectors.Types}.DataCollector

Implements Issue 870: inst-status-xen should not be shown for KVM
cluster.

Signed-off-by: Aaron Karper <akarper@google.com>
Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>