Fix bug when attempt to create fip in a private net

This commit fixes an Exception when user tries to create a
floating ip on a no-external network.

Closes-Bug: 1322185
Signed-off-by: Sahid Orentino Ferdjaoui <sahid.ferdjaoui@cloudwatt.com>
Merge pull request #17 from sahid/master

Fix bug when attempt to create fip in a private net