Previously the code looked at mariadb.pid, but this seemed
flakey in function. It seemed racey and prone to failure on
slower connections to a registry. The original task was
extremely complex and it didnt really verify that the MariaDB
was ready to serve connections. Use wait_for with a regex
instead.
Change-Id: I3aafac04f03639b08e0ef4d6a9c9e1a4499f000c
Closes-Bug: #1564278