With netaddr.valid_ipv4/6, empty host is not allowed and will fail
to validate.
This patch changes to the default wsgi listen host to "0.0.0.0"
and enable to validate it by using netaddr.
Note: The default behavior is NOT changed.
Signed-off-by: IWASE Yusuke <iwase.yusuke0@gmail.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>