r/useproxy: Configure system-wide proxy

The *useproxy* role configures the `http_proxy` et al. environmet
variables for systemd services and interactive shells.  Additionally, it
configures Yum repositories to use a single mirror via the `baseurl`
setting, rather than a list of mirrors via `metalink`, since the proxy
a) the proxy only allows access to _dl.fedoraproject.org_ and b) the
proxy caches RPM files, but this is only effective if all clients use
the same mirror all the time.

The `useproxy.yml` playbook applies this role to servers in the
*needproxy* group.

roles/useproxy/handlers/main.yml

@@ -0,0 +1,6 @@
+- name: reload systemd
+  systemd:
+    daemon_reload: true
+
+- name: reset connection
+  meta: reset_connection