Remove network_type slot (Issue 363)

This slot was not used by Ganeti so the same info can be
provided via tags. In order not to break configuration data
we add a FromDict() method in Network config object that
removes the deprecated network_type (if found) and then invoke
the parent's method (which does the validation).

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Reviewed-by: Guido Trotter <ultrotter@google.com>