Fixed bug that allowed to use a local address for static route

Added ifaceByAddress method in network for this; the replacement of IPAddressExists method has fixed some bug in interface address (you could set a iface address in the loopback network for example)

Changed also EBox::Exceptions::DataExists to allow better message preserving the exception class (the user is notified of what iface has the same address when trying to set a repeated address for a iface)