(FACT-697) Add confine to dhcp_servers for nmcli

This patch adds a confine on dhcp_servers to ensure that, not only is
nmcli present, but actually is connected to NetworkManager.

This fixes the issue where facter is throwing an error of
'NetworkManager is not running' at each run of facter.

Closes: FACT-697
Merge pull request #811 from onyxpoint/FACT-697

(FACT-697) Add confine to dhcp_servers for nmcli