Permit overriding the mysql InnoDB pool size.

For VMs we need a small pool - e.g. 100MB. For production environments
folk will likely want a much larger pool size. Ideally we would set
The Right Value automatically, but as an interim step, make it
possible to set a custom value again in devtest_*.

Change-Id: Id595113a52f5a1da740bd81a4cf7932c96d57754
Merge "Permit overriding the mysql InnoDB pool size."