(FACT-593) Remove dependency on route binary from dhcp_servers fact

Previously, the dhcp_servers fact assumed that the route binary
was present, which it then tried to use to determine the primary
interface. This commit updates that fact to instead use /proc/net/route
to retrieve the same information.
Merge pull request #699 from whopper/dhcp_route_fix

(FACT-593) Remove dependency on route binary from dhcp_servers fact