Set MySQL Buffer Pool size low for testing

In devtest, we need this to be very low as we expect small VMs. But in
production we will want a much larger default. We need to be explicit
here, so that we can raise the default in the element.

Change-Id: Ia3cbffa9e795d22e8eddcc44db7d1548e8d99c91
Merge "Set MySQL Buffer Pool size low for testing"