Update mysql to 5.7.20
Update image to version 5.7.18-r2
**Description of the change** Use indirect reference in the check_for_deprecated_env function. This fixes a bad usage of indirect references. This prevents the `app-entrypoint.sh` from prompting the user when the deprecated env vars are not set.
This release adds the possibility to enable/disable blank passwords. Disabled by default
Bitnami Bot
Joseda Rios
Sameer Naik