Fix empty UDP pools status: a UDP pool without members went OFFLINE
instead of ONLINE.
This commit changes the keepalived configuration with empty pools:
the configuration now contains a virtual_server and a comment about the
existing pool. This comment is used by the get_udp_listener_pool_status
to detect that the pool exists and is not offline.
Story 2007984
Task 40610
Conflicts:
octavia/tests/unit/common/sample_configs/sample_configs_combined.py
Change-Id: I30e23ca13d033d77c8ebdabbfdc7b54556a9466b
(cherry picked from commit 2954370e38)