Add subnet_id option

When tested network contained more than one subnet then test failed
due to assert call.

Subnet_id option specifies which subnet should be used for testing
when more than one subnet in network is present. If subnet_id is
specified then each created VM is assigned a floating IP within
the range of the subnet.

Closes-Bug: 1856671
Change-Id: Ie4d44b16595055201e12f10982fc78683a12d41a
4 files changed