Create subnet without gateway and explicit IP ver

Now it's impossible to create subnet without gateway in network
tests. This patch allows you to set gateway explicitly to None.
Backward compatibility is supported: by default it creates
subnet with default gateway as before. Also it adds possibility
to create subnet with specific IP version when you need to create
two subnets in one tenant of different IP version ("dual-stack").
Fixed attributes test for new requirements and added 2 anothers.

Change-Id: I7aca5e07be436f20cba90339785b46182d97fead
Fixed a comment on dirty logs list

We've recently changed default behaviour to forbid any dirty logs for
services outside the list instead of explicitly enabling log checks for
specific services. The comment was not updated to reflect the change
though.

Change-Id: I5d5ba64d156a9b5318cfc2740b2a69d75de3c127
Merge "Create subnet without gateway and explicit IP ver"
Merge "Fixed a comment on dirty logs list"
Merge branch 'master' of https://github.com/openstack/tempest