Set database collation

Set database collation to match upstream settings. Related to https://github.com/gitlabhq/gitlabhq/pull/8195.
I think `attributes/default.rb` should use the Postgres defaults. @bbodenmiller you could add the collation setting to the [MySQL configuration example](https://github.com/gitlabhq/omnibus-gitlab/blob/master/doc/settings/database.md#configure-omnibus-gitlab-to-connect-to-it)