Revert "Install dnf on CentOS hosts"

This reverts commit 7401dddaa2.

CentOS has removed dnf from the EPEL repositories and will have
an updated dnf available for CentOS 7 sometime soon. ☹

Change-Id: I888826371930517a722e4625f68f023e28bdb3d2

playbooks/openstack-hosts-setup.yml

@@ -24,15 +24,12 @@
   gather_facts: false
   user: root
   pre_tasks:
-    - name: Install prerequisites before setting up the host
+    - name: Ensure python is installed
       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