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] 697cbb83dbd538c7f51638f50531dac6a4b518c0
Change-Id: I20dfe31776251be63eac1b69501f1714fdae40d7
da8f422e3b