Code review comment for lp://qastaging/~marcoceppi/charms/trusty/mysql/lp1373862

Please see comment on bug 1373862.

If we do move forward with this default value change, we will need to update the amulet test as well.

The test knows about the former 80% default charm config value, in the interest of making sure the "deployed" == the "expected."

As an aside, the unit tests are known-failing (bug 1398918), as they're not really mocking.