Ubuntu: Use utf8mb3_general_ci collate in MySQL

Currently idempotency in Ubuntu is broken because of the below change
detected in collate in MySQL.

```
/Stage[main]/Keystone::Db::Mysql/Openstacklib::Db::Mysql[keystone]/
Mysql_database[keystone]/collate: collate changed 'utf8mb3_general_ci'
to 'utf8_general_ci'
```

Similarly to what we observed in the past about charset[1], it seems
MySQL in Ubuntu is automatically converting the collate value and that
is causing the "unexpected" change detected in the 2nd puppet run.

This fixes the idempotency by using utf8mb3_general_ci in Ubuntu to
avoid the mismatch caused by internal translation.

[1] 697cbb83db

Change-Id: I20dfe31776251be63eac1b69501f1714fdae40d7

spec/acceptance/mysql_spec.rb

@@ -17,6 +17,7 @@ describe 'openstacklib mysql' do
 
       openstacklib::db::mysql { 'ci':
         charset       => $charset,
+        collate       => "${charset}_general_ci",
         password_hash => mysql::password('keystone'),
         allowed_hosts => '127.0.0.1',
       }