bootstrap: DNF is the default package manager on Fedora > 22
bootstrap: DNF is the default package manager on Fedora > 22

Let's use dnf if the fedora release is 23 or greater unless let's fall
back to yum

Signed-off-by: Erwan Velu <9949cc68615082c237e6af925cacbd2ea7f2e953@redhat.com>

dmick/teuthology,ceph/teuthology,dmick/teuthology,SUSE/teuthology,dmick/teuthology,ktdreyer/teuthology,ceph/teuthology,SUSE/teuthology,SUSE/teuthology,ktdreyer/teuthology