Add doc on ssh key replacement if repackaging (#1169)

If planning to repackage a machine that is brought up with vagrant and
modified, it is necessary to avoid the default ssh replacement that
normally takes place to ensure the subsequent box that is halted and
packaged can be accessed by vagrant using the default ssh key
subsequently.

Add some small documentation notes to help users be aware to this so
that they know to set `config.ssh.insert_key = false` when using vagrant
to build a box.

Fixes: #1116

tools/create_box.sh

@@ -11,6 +11,10 @@ usage() {
     echo "Usage: ${0} IMAGE [BOX] [Vagrantfile.add]"
     echo
     echo "Package a qcow2 image into a vagrant-libvirt reusable box"
+    echo ""
+    echo "If packaging from a Vagrant machine ensure 'config.ssh.insert_key = false' was "
+    echo "set in the original Vagrantfile to avoid removal of the default ssh key, "
+    echo "otherwise vagrant will not be able to connect to machines created from this box"
 }
 
 # Print the image's backing file