Resolve "hammer.py sometimes cannot start mariadb service on alpine"

Closes #3547 (closed).

• a08efdba hammer.py: wait for start-stop-daemon/mysqld_safe to exit

  Attempt to fix start-stop-daemon: /usr/bin/mysqld_safe is already running

• https://jenkins.aws.isc.org/job/kea-manual/job/hammer-and-configure/91/