Allow adding extra fields to all discovered entities

[mohabusama]

We can accept extra set of fields that will be added to all discovered entities.

Example:

EXTRA_ENTITY_FIELDS="zmon_environment:staging"

Those fields will affect how we sync the entities.

[mikkeloscar]

We would very much like to have this. Our use case is similar to your example e.g. having an environment field to distinguish between production and test environments in alerts.