Properly default MysqlInnodbBufferPoolSize (undercloud) v2

I botched the first version of this but it merged. jq doesn't deep merge
on +, so . completely overrides .parameters from the defaults object.

This solution works by doing the addition at the level of the parameters
object. At this level, a shallow merge is correct.

Fixes Ic27513170df3ef15ebdf3b3e6f07527a30e656ea

Change-Id: I42a9d4b85edcc99d13f7525e964baf214cdb7cbf
Merge "Properly default MysqlInnodbBufferPoolSize (undercloud) v2"