It seems as if the current image for MySQL is not too big for its
flavors of test.small-3. These need to be changed to test.small-4 to fix
the gate.
The defaults for PXC and Percona were also changes, just to make
sure (MariaDB was already there).
Change-Id: Id5be526a0418db5d00cbf5cbceb4993274989e72
Closes-Bug: #1656707
e6ee6062df