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/defaults/main.yml

@@ -11,3 +11,4 @@ dhcpcd_noipv4ll: false
 dhcpcd_noipv4: false
 dhcpcd_noipv6rs: false
 dhcpcd_interfaces: []
+dhcpcd_nodev: false