(FACT-697) Fix nmcli command line usage for versions < 0.9.9.

The code to confine the dhcp_servers fact to when NetworkManager's
status is not "unknown" used a command line syntax that is only
available in nmcli versions >= 0.9.9.

This commit uses the older command line syntax when the nmcli version is
less than 0.9.9.
Merge pull request #818 from peterhuene/fact/697

(FACT-697) Fix nmcli command line usage for versions < 0.9.9.