Use utf8 character set as default

[srna]

I believe it is reasonable to use utf8 as the default charset in MySQL databases. This PR makes the dokku plugin use this charset as default.

[josegonzalez]

Fuck yeah utf-8!

How does this work with databases where:

• the database is created and is running
• the container is stopped and removed
• the plugin is updated to this version with utf8 defaults
• the container is started again

[aminotsukasa]

It is simple to use utf-8 charset.

[srna]

@josegonzalez, I think it only affects database creation, so it doesn't have effect on other states of the database/container lifecycle.