Require that deltarpms be v3 and signed

[DemiMarie]

v3 deltarpms can be signed, and libdnf should verify the signature before passing them to drpm. The payload digest will be wrong, but that is okay since the header+payload signature can still be validated. This means that header+payload signatures will be required for deltarpms.

[j-mracek]

I am really sorry but I do not know what we can do with it or what plans are for deltarpm in future. We believe that verification will be not easy and we would prefer if a library or deltarpm will do it for us.

[DemiMarie]

@j-mracek deltarpm does not have any signature verification functionality. Verification can be handled by librpm itself, as with normal RPMs.