Defer to element mysql.innodb_buffer_pool_size

Choosing 100MB here is not a production default. We also don't need two
places with the default value set. The closer a default is to the actual
usage of it, the better, so we'll set 0 here, which will defer to the
default in the element.

Change-Id: I1b41b604286245c2fb83249778db835253c02fc5