Fix multipool tests broken by EventletDeprecationWarning in stdout

designate-manage imports oslo_concurrency, which detects eventlet (still installed as a transitive dependency via oslo.service, nova, etc.) and emits an EventletDeprecationWarning to stderr.

The _run_designate_manage helper was using stderr=subprocess.STDOUT, merging stderr into the captured stdout. This corrupted the YAML output that the tests parse, causing 4 multipool test failures.

Switch to stderr=subprocess.PIPE so that stderr is captured separately and does not interfere with stdout parsing. Log stderr on CalledProcessError to preserve debuggability.