Allow use of dnf instead of yum on Fedora

Since Fedora 20 it has been possible to use 'dnf' as a drop-in
replacement for 'yum', and it is targetted to become the default
in Fedora 22

   http://fedoraproject.org/wiki/Changes/ReplaceYumWithDNF

There are many benefits of 'dnf' over 'yum' but the biggest
from the POV of an openstack developer is its speed.

Assuming an existing running devstack install ie all required
RPMs already installed on the system. Now look at how long it
takes to run stack.sh, during which yum does not have to
actually install anything

 # ./unstack.sh
 # time ./stack.sh
 real 11m12.193s
 user 10m17.129s
 sys  0m15.275s

Now, with 'export YUM=dnf' set in local.conf, run the same
test again

 # ./unstack.sh
 # time ./stack.sh
 real 0m48.610s
 user 0m28.939s
 sys  0m7.801s

So, this is showing that devstack is wasting 10 minutes just
for yum to figure out that everything is already installed.
The overhead of yum vs dnf is even worse when yum has to
acutally depsolve to install new packages.

Change-Id: Ia01a5f330a47b32207586902a861bedfc8a0f6e2
Merge "Allow use of dnf instead of yum on Fedora"