Fix password check in post-mysql-install

If we look at the code, we can see that post-mysql-install just greps .my.cnf for a password. This is likely to lead to problems in a bunch of scenarios, such as if someone has multiple configuration sections with different passwords defined. See here:

https://github.com/mariadb-corporation/mariadb-columnstore-engine/blob/893cca974df93478050ff23b322a97e5f3a1e78e/oam/install_scripts/post-mysql-install#L13

We should probably make this check more robust. Could it be changed to use my_print_defaults or something?

https://mariadb.com/kb/en/library/my_print_defaults/