Allow gateway address to be unset for an existing subnet

Validation of the gateway IP address was taken place even
if the gateway IP was set to None; this was causing an
AddrFormatError exception to be raised by method
'_validate_gw_out_of_pools'. With this change we skip
the validation in case the gateway is set to None.

Fixes bug #1178273

Change-Id: Ib84378a4fd2cefdbbcacce695abbfaf82c647dd3