added ifaceByAddress method. Don't allow to use a local address as gateway address
More flexibility for printable message in EBox::Exceptions::DataExists
Adapted to use of ifaceByAddress, removed IPAddressExists
added address parameter validation for EBox::Network::ifaceByAddress
Merge pull request #285 from Zentyal/jag/network-gw-nolocalhost

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