r/useproxy: Configure dnf to use proxy

Although running `dnf` from the command line works without explicitly
configuring the proxy, because it inherits the environment variables set
by PAM on login from the user's shell, the `dnf` Ansible module does
not, as it does not inherit those variables.  Thus, we need to
explicitly configure the `proxy` setting in `dnf.conf` in order to be
able to install packages via Ansible.

Since `dnf` does not have separate settings for different protocols
(e.g. HTTP, HTTPS, FTP), we need a way to specify which of the
configured proxies to use if there are multiple.  As such, the
*useproxy* role will attempt to use the value of the `dnf_proxy`
variable, if it is set, falling back to `yum_proxy` and finally
`http_proxy`.  This should cover most situations without any explicit
configuration, but allows flexibility for other cases.

roles/useproxy/tasks/main.yml

@@ -49,6 +49,16 @@
   tags:
   - systemd
 
+- name: ensure dnf is configured to use proxy
+  ini_file:
+    path: /etc/dnf/dnf.conf
+    section: main
+    option: proxy
+    value: '{{ dnf_proxy|d(yum_proxy|d(http_proxy)) }}'
+    state: present
+  tags:
+  - yum
+
 - name: ensure yum repos are configured to use baseurl
   ini_file:
     path: /etc/yum.repos.d/{{ item.file }}.repo