Fix ipv6 network scenario in multi-network environment

This commit makes the server creation as part of the testing use the
network created for the test. This is required if there is more than
1 network for the tenant used by the test. Which is normally the case
outside of running with tenant isolation.

Closes-Bug: #1401961

Change-Id: Ia5e177ad1124f9ee6cd7e92fad614f3310e1a86f
Merge "Fix ipv6 network scenario in multi-network environment"