Use master IP address setup script in backend

Replace the code in backend.ActivateMasterIp and
backend.DeactivateMasterIp with the master IP address setup script,
either the default one or the one provided by the user.

- Convert to string the netmask parameter in _BuildMasterIpEnv
- Add the _RunMasterSetupScript function and let ActivateMasterIp and
  DeactivateMasterIp become wrappers for it;
- Change the failure model of DeactivateMasterIp, raising errors if the
  script fails instead of silently ignoring the failure

Signed-off-by: Andrea Spadaccini <spadaccio@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>