Fix tests to honor Octavia API versioning

This patch adds support for Octavia API endpoint versioning.
Previously Rocky API additions were being tested, regardless of the Octavia
API version.
It also fixes a potential race condition when testing the "ONLINE"
operating status.
The patch also makes sure that duplicate fixed IP addresses are not allocated
on subnets.

Story: 2003466
Task: 24726

Change-Id: If0ec777f34326c09c7215d9c7c2ed249571d779e
9 files changed