Add final dnf support bits

The dnf package manager was missing from the list of package
managers in the upgrade check tasks. This patch ensures that
the upgrade check runs on servers with dnf.

The dnf module has no 'update_cache' argument, so this patch removes
that if dnf is being used. Updating the metadata cache is not needed.

Implements: blueprint centos-and-dnf
Change-Id: I10565b2e9379d5ac60ae65bfcdda221a0218988e

tasks/install_yum.yml

@@ -103,7 +103,7 @@
   package:
     pkg: "{{ rabbitmq_dependencies }}"
     state: "{{ rabbitmq_package_state }}"
-    update_cache: yes
+    update_cache: "{{ ansible_pkg_mgr == 'yum' | ternary('yes', omit) }}"
   register: install_packages
   until: install_packages|success
   retries: 5

tasks/rabbitmq_upgrade_check.yml

@@ -39,7 +39,7 @@
   register: installed_rabbitmq_rpm
   when:
     - not rabbitmq_upgrade | bool
-    - ansible_pkg_mgr in [ 'yum', 'zypper']
+    - ansible_pkg_mgr in ['yum', 'dnf', 'zypper']
   tags:
     - rabbitmq-package-rpm
     - rabbitmq-yum-packages