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.

group_vars/Fedora.yml

@@ -0,0 +1,10 @@
+useproxy_yum_repos:
+  - file: fedora
+    name: fedora
+    baseurl: http://dl.fedoraproject.org/pub/fedora/linux/releases/$releasever/Everything/$basearch/os/
+  - file: fedora-cisco-openh264
+    name: fedora-cisco-openh264
+    baseurl: https://codecs.fedoraproject.org/openh264/$releasever/$basearch/os/
+  - file: fedora-updates
+    name: updates
+    baseurl: http://dl.fedoraproject.org/pub/fedora/linux/updates/$releasever/Everything/$basearch/