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] f29a3051fc

Depends-on: https://review.opendev.org/851775
Change-Id: I0fddef098e72d515ef746af56b98bbf190896214

spec/acceptance/10_basic_manila_spec.rb

@@ -40,6 +40,7 @@ describe 'basic manila' do
       }
       class { 'manila::db::mysql':
         charset  => $::openstack_integration::params::mysql_charset,
+        collate  => $::openstack_integration::params::mysql_collate,
         password => 'a_big_secret',
       }
       class { 'manila::keystone::auth':