fix 1208: add native charm support for rhel and fix centos support

The commit adds SO discovery support for rhel and adds a script to
add a DNAT rule to rhel and centos images.

There are some things that needs to be taken in acount.
- It uses firewalld, wich is the default firewall for centos and rhel,
  instead of iptables. This may break some thinghs if the image uses
  iptables.
- If firewalld is not actibe it assumes that the VM is not using it,
  so it creates a input rule to accept all incoming conections
  (by default rejects all). If the firewall was not enabled, with
  accept all it should have the same behavior.

Change-Id: I29c9781d354c4e8268e19f64dcc9568d725a0de3
Signed-off-by: endika <endika.aldecoa@ehu.eus>