CentOS: Stop installing yum

CentOS/RHEL replaced yum by dnf and yum is no longer available by
default. We (and the other projects) updated implementations to adapt
that change so we assume we no longer need the old yum command (which
is just an alias of dnf).

Depends-on: https://review.opendev.org/884307
Change-Id: I5152d1013a603e8d230748ff956d33507675450c

playbooks/prepare-node-common.yaml

@@ -43,11 +43,6 @@
           - zlib-devel
           - rubygem-rexml
 
-      - name: Install yum (CentOS/RHEL)
-        dnf:
-          name: yum
-        become: true
-
       when:
         - ansible_os_family == 'RedHat'
 
@@ -95,12 +90,12 @@
 
     - block:
       - name: Set up puppetlabs repo (CentOS/RHEL)
-        yum:
+        dnf:
           name: "https://yum.puppetlabs.com/puppet{{ puppet }}-release-el-{{ ansible_distribution_major_version }}.noarch.rpm"
         become: true
 
       - name: Install puppetlabs puppet-agent (CentOS/RHEL)
-        yum:
+        dnf:
           name: puppet-agent
         become: true