(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.
Looks like we can safely return nil here where it's currently called in facter, except for the added call to the spec in this commit needs a `should be_nil` instead.