Install dnf on CentOS hosts

This patch adds dnf to CentOS hosts to provide better package
manager performance.

Implements: blueprint centos-and-dnf
Change-Id: Ieea80e88652dd84851aeed9a1910f86ec7ba86d0

playbooks/openstack-hosts-setup.yml

@@ -24,12 +24,15 @@
   gather_facts: false
   user: root
   pre_tasks:
-    - name: Ensure python is installed
+    - name: Install prerequisites before setting up the host
       register: result
       raw: |
-        if which apt-get >/dev/null && ! which python >/dev/null ; then
+        if which apt-get > /dev/null && ! which python > /dev/null ; then
           apt-get -y install python
           exit 2
+        elif which yum > /dev/null && ! which dnf > /dev/null ; then
+          yum --disablerepo=extras -y install dnf dnf-plugins-core python-dnf
+          exit 2
         else
           exit 0
         fi