rpm: increase min required Fedora to 27
rpm: increase min required Fedora to 27

Fedora 26 has reached its EOL on May 29th 2018.

https://lists.fedoraproject.org/archives/list/announce@lists.fedoraproject.org/message/XGG7GYUJN3VDA6HX4KJNFSKSQCGF2FMW/

Signed-off-by: Fabiano Fidêncio <2931bae082ef9bc04523ba759c03a4778316d48f@redhat.com>
Reviewed-by: Erik Skultety <2c14d38fa47c8799f1b9c16280abe27f8edfec6e@redhat.com>

andreabolognani/libvirt,olafhering/libvirt,jardasgit/libvirt,nertpinx/libvirt,eskultety/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,libvirt/libvirt,jfehlig/libvirt,zippy2/libvirt,eskultety/libvirt,crobinso/libvirt,olafhering/libvirt,libvirt/libvirt,nertpinx/libvirt,nertpinx/libvirt,libvirt/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,eskultety/libvirt,jfehlig/libvirt,jfehlig/libvirt,crobinso/libvirt,crobinso/libvirt,andreabolognani/libvirt,zippy2/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,libvirt/libvirt,andreabolognani/libvirt,olafhering/libvirt,eskultety/libvirt,andreabolognani/libvirt,crobinso/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,jardasgit/libvirt,jardasgit/libvirt,olafhering/libvirt,zippy2/libvirt,eskultety/libvirt,jardasgit/libvirt,jfehlig/libvirt,zippy2/libvirt