Add option to use user-defined Collector template

[vgratian]

Is your feature request related to a problem? Please describe.
Currently they way poller loads and merges the templates of a Collector is hardcoded:

• First, poller loads conf/<collector>/default.yaml
• Second, if exists, it loads conf/<collector>/custom.yaml and merges to default.

Users can create conf/<collector>/custom.yaml and add additional objects they want to collect. But they can't:

• disable objects that are already defined in conf/<collector>/default.yaml
• create multiple custom templates and apply to different pollers

Of course, it is possible is to customize conf/<collector>/default.yaml, but this way changes need to be re-applied after each update of Harvest.

Describe the solution you'd like
Add the option to add list of (custom) templates in the collectors: parameter in the Pollers: section of harvest.yml.

Describe alternatives you've considered
Another possible solution is to add a Collectors: section to harvest.yml where you define different configurations of the same collector in the same way we do with exporters. But I think this is much more work than adding a single parameter.

Additional context
In Harvest 2.0 Alpha (Python version) a similar parameter was implemented. The collectors: section could be expanded to with any (valid) collector parameters. E.g.:

Pollers:

  shopfloor:
    datacenter: Walldorf
    url: 10.0.0.2
    exporters:
      - prometheus01
    collectors:
      Zapi:
        templates:
           - custom2.yaml

[cgrinds]

Covered by #555 and #505