allow setting enable_functions for mysql databases on upgrade

[markdboyd]

Changes proposed in this pull request:

Currently, we allow users to set enable_functions when creating new MySQL databases, but not when updating them. So users with existing MySQL databases can't enable this feature without creating a new database and importing a backup into it.

This PR enables the ability to set enable_functions when updating a database service, so customers with existing MySQL databases can easily enable this feature.

Things to check

• For any logging statements, is there any chance that they could be logging sensitive data?
• Are log statements using a logging library with a logging level set? Setting a logging level means that log statements "below" that level will not be written to the output. For example, if the logging level is set to INFO and debugging statements are written with log.debug or similar, then they won't be written to the otput, which can prevent unintentional leaks of sensitive data.

Security considerations

None, just exposing an option on update that is already exposed on creation

[dandersonsw]

Awesome, thanks Mark!