QA: Release instance in ganeti-qa

Release instances directly from “ganeti-qa” instead of doing so in the
three tests removing instances. This is in line with how nodes are
released and makes it easier to find and debug double-release issues.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>