Description of problem:

Would it be possible to make `$wsrep_provider_options` in `https://opendev.org/openstack/puppet-tripleo/src/branch/master/manifests/profile/pacemaker/database/mysql_bundle.pp` a hash and collaps it to a string after the `deep_merge` in line 346? 
I think this would make it possible to set individual `wsrep_provider_options` without messing around with `gmcast.listen_addr` or potential tls settings. 

Reason: 
Because of the high instance create/delete rate in some of our environments the default `gcache.size` of 128mb leaves us with maintenance windows of less than 30 minutes. 
Hardware replacements regularly take us more than one hour. 
To avoid SST we increased the `gcache.size` see attached `mysql.yml`[1].  
The `gmcast.listen_addr` isn't a parameter we want to change but as far as i can tell there is currently not way to set an individual option because `wsrep_provider_options` is a string. 

[1] mysql.yml
~~~
parameter_defaults:
  ControllerExtraConfig:
    tripleo::profile::base::database::mysql::mysql_server_options:
      mysqld:
        wsrep_provider_options: "gcache.size=512mb;gmcast.listen_addr=tcp://%{tripleo::profile::pacemaker::database::mysql_bundle::gmcast_listen_addr}:4567;"
~~~

We need to set gcache.size to an appropriate value to prevent SSTs from happening when we need to reboot a single controller.
We would like to do that by simply setting wsrep_provider_options and having that merged with the default config instead of overwriting.

The issue we are trying to fix with the above mentioned procedure happens in OpenStack regions with high instance ceate/delete rate during maintenance.

If you know of a different clean way to avoid an SST/increase `gcache.size` i would be happy to give it a try.