Support for custom add custom labels from reclass

This change will enable possibility to add custom
labels to alerts.
This can be used ex. to provite routing key for
pushkin (notification service)

If given key already exists in alert, it will NOT
be overriden.

prometheus:
  server:
    config:
      alert:
        labels_add:
          route: 'mail,skype'

Change-Id: I0b21d8a9695e8a15d0d6b80e6ddd0476682c80c2
1 file changed