We need to support different report types for OpenScap, ABRT and other future reporting tools.

This means there must be a way to register new report types and importers from plugin (exactly the same we have for facts now). Reports will probably only differ in metrics and their own statuses, therefore we should make metrics optional and mapping to global status should be different per specific status.

There should be a generic API for uploading reports. Common things for all types are:
- message (text, status, level)
- reported_at
- status (custom hash of format "status": n, each type different statuses)

User stories:
- As a plugin developer I want to register new report type.
- As a plugin developer I want to register new report importer for a given type.
- As a user I want to search hosts based on report type and its specific status.
- As a user I want to have be able to set different report permissions per report types. (implies report STI)