Fix scenario tests when default-subnetpools extension is absent

On some networking backends like Tungsten Fabric subnetpools
extensions are not supported, so usage of related arguments
when creating subnet should be avoided.

Related-Prod: https://mirantis.jira.com/browse/PRODX-49702
Change-Id: Ia76085810c34c1a0af064e137440b02d3edc9abe
1 file changed