Allow to set charset in nova::db::mysql

The charset `latin1` is hardcoded in `nova::params::nova_db_charset`.

I would like to be able to change the default charset like it is possible with all other puppet modules for Openstack.

Is there any reason or known limitations/problems that could explain why it's hardcoded to `latin1`? (limitations from Openstack/Nova itself for example)

If not, I will go ahead and patch `nova::db::mysql` to support a new parameter called `charset` with a default value set to `latin1`.