Disabled accept integer as ipv4 "Netmask".

As accepts the integer,the nic will not come up. When switching it back to
the full netmask it works fine.
So now only accept IPv4Address or empty for "Netmask" of ipv4.

Change-Id: I68b20ea199826532626413ed55467d49d61a58cd
Signed-off-by: hadong <hadong0720@gmail.com>
Bug-Url:https://bugzilla.redhat.com/show_bug.cgi?id=990965