tools: engine-setup - fixed installation messages

Display instructions for configuring iptables and firewalld
only if they're available on the system.

Change-Id: I89964bc30c275a29428abef06d07b7a8911ee211
Bug-Url: https://bugzilla.redhat.com/921079
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>