[service discovery] fix fallback IP address extraction

[hkaj]

Note: Please remember to review the Datadog Contribution Guidelines
if you have not yet done so.

What does this PR do?

When no key (network name or id) was passed to the host variable, we used to skip the _get_fallback_ip method and only try using the Config.NetworkSettings.IPAddress address (which is empty more often than not now). With this PR, _get_fallback_ip applies every time, using the bridge network address or the last network (sorted) address.

Motivation

More and more often Config.NetworkSettings.IPAddress is empty, relying on it makes the agent fail in cases where it could infer the config automatically.

Testing Guidelines

Updated the relevant mocks in unit tests

Additional Notes

• Fix [Service Discovery] %%host%% and %%port%% don't seem to work #2845
• Also limit the split('_') to 1 to avoid splitting the specifier if it has a _.

[masci]

LGTM, there was a flaky test, restarted and now it's green. 👍