tmt tests export --nitrate: Allow case creation

[lukaszachy]

When I am writing new test, I want to be able specify everything using tmt/fmf and let tmt create the test case on nitrate server.

Otherwise for new test I would need to manually (or by other tool) create some test case, and either manually create the tcms attribute in fmf file, or let the tmt tests convert import it.

[lukaszachy]

@psss What are you thoughts about this? Might be handy for adoption.

[psss]

Agreed, this might be handy during the transition period when we still need nitrate test cases. The tmt test export seems the best place. It could be something like this:

tmt test export --format nitrate --create

[hegerj]

@psss How can I determine which file defines test case (in order to write to it newly created extra-nitrate id).

[psss]

Each fmf node object keeps the list of files from which metadata has been constructed in sources. If I remember correctly, the order should follow the inherit steps so the last item from the list should be your friend :)

[psss]

Implemented by #185.