roles/dhcpcd: Disable device management module

Today I realized that `dhcpcd` has been logging several hundred thousand
of these messages every second:

    libudev: received NULL device

This was causing both `dhcpcd` and `systemd-journald` to consume 100%
CPU.

I am not entirely sure what a "device management" module is in the
context of `dhcpcd`, but it does not seem to be required. Setting the
`nodev` option in `dhcpcd.conf` suppresses the messages, and seems to
have no effect on the operation of the daemon.

roles/dhcpcd/templates/dhcpcd.conf.j2

@@ -1,5 +1,10 @@
 # A sample configuration for dhcpcd.
 # See dhcpcd.conf(5) for details.
+{% if dhcpcd_nodev %}
+
+# Disable device management to avoid "libudev: received NULL device" log spam
+nodev
+{% endif %}
 
 # Allow users of this group to interact with dhcpcd via the control socket.
 {% if dhcpcd_controlgroup is defined %}