Skip caching for dnf (Fedora 22) builds

We are still working on the caching story for dnf-based builds
(i.e. Fedora 22). There are a couple of options which we will work
through on the linked page.  We will sort this out before we move the
devstack job (yet to be created, because the nodes aren't there yet)
out of experimental.

In the mean time, disable the caching in these elements so we can get
Fedora 22 image builds (currently they're in a big looping failure
[1]).

[1] http://nodepool.openstack.org/image.log

Change-Id: I3a435889fc5109d7365240068047aac98abc605e

nodepool/elements/cache-bindep/install.d/50-cache-pkgs

@@ -15,6 +15,12 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+if [[ ${YUM:-yum} == "dnf" ]]; then
+    echo "*** caching disabled for dnf"
+    echo "*** see https://etherpad.openstack.org/p/infra-f22"
+    exit 0
+fi
+
 # Build up a list of packages to install by concatenating bindep lists
 FALLBACKREPO=/opt/git/openstack-infra/project-config
 FALLBACKFILE=jenkins/data/bindep-fallback.txt

nodepool/elements/cache-devstack/install.d/50-download-pkgs

@@ -17,6 +17,12 @@
 
 set -e
 
+if [[ ${YUM:-yum} == "dnf" ]]; then
+    echo "*** caching disabled for dnf"
+    echo "*** see https://etherpad.openstack.org/p/infra-f22"
+    exit 0
+fi
+
 if grep "^CentOS release 6" /etc/redhat-release ; then
     # --downloadonly is provided by the yum-plugin-downloadonly package on
     # CentOS 6.x