roles/munin: Use apt module explicitly

Instead of using dynamic hack to use the package manager for the
current host. We only have Ubuntu here anyways.

Signed-off-by: Alan Orth <alan.orth@gmail.com>

roles/munin/tasks/munin-node.yml

@@ -1,6 +1,7 @@
 ---
 - name: Install munin-node
-  action: "{{ ansible_pkg_mgr }} name={{ item }} state=latest"
+  apt: name={{ item }} state=latest
+  when: ansible_distribution == "Ubuntu"
   with_items:
     - munin-node
 

roles/munin/tasks/munin.yml

@@ -1,6 +1,7 @@
 ---
 - name: Install munin package
-  action: "{{ ansible_pkg_mgr }} name={{ item }} state=latest"
+  apt: name={{ item }} state=latest
+  when: ansible_distribution == "Ubuntu"
   with_items:
     - munin