Cloned from https://bugzilla.redhat.com/show_bug.cgi?id=1649800

Description of problem:
When generating a report (as in clicking "Generate"), we warn a user about resource consumption implications and want to confirm they really want to generate the report.

While generating a preview of the report is much easier to reach (whenever you edit a template, you tend to preview it before submitting and the button is readily available), we don't issue any such warning there.

The warning should either be issued OR we could limit a number of hosts loaded, thus limiting the dataset we work with. The operation would be cheaper, thus suitable for preview without warning. However, the user needs to be well aware that the result is just a sample. This could also lead to selecting such a dataset that this sample would be useless for the user (e.g. empty).

Version-Release number of selected component (if applicable):
Sat 6.5

How reproducible:
Deterministic

Steps to Reproduce:
1. Monitor -> Report Templates -> <report> -> Preview

Actual results:
No warning, full report generated

Expected results:
Either warning or limited dataset, after thinking through the implications. Or something completely different that would make sense.