CLOUDSTACK-1088: EnableStaticNat error will clear the data in database

The issue occur in two conditions

(1) If I use two sessions or browsers to EnableStaticNat on CloudStack
UI. one is successful, the other is failed. However, there is no ip in
database.

(2) If I use API call EnableStaticNat several times The first time
succeed, the second failed, the third succeed. the result is
success-fail-success-fail-success-fail, which it is not correct.

Reported-by: Wei Zhou <w.zhou@leaseweb.com>
Reviewed-by: https://reviews.apache.org/r/9254/
Signed-off-by: Prasanna Santhanam <tsp@apache.org>