Support major MySQL version upgrade in the operator

[frouioui]

We should be able to support major MySQL version upgrade in the vitess-operator. #361 adds support for minor version upgrades.

The issue with doing a major version upgrade lies in the different collations used by MySQL 5.7 and 8.0. During the upgrade process, VReplication fails because of the default collation of the primary mysqld (mysql8.0) does not exists on the replica mysqld (mysql5.7).