XtraDB Cluster support for replication over SSL

Allows you to adjust `wsrep_provider_options` for encrypted Galera replication. 

Example:

```ruby
default_attributes(
  "percona" => {
    "server" => {
      "role" => "cluster",
    },

    "cluster" => {
      "package"                => "percona-xtradb-cluster-55",
      "wsrep_provider"         => "/usr/lib/libgalera_smm.so",
      "wsrep_provider_options" => '"socket.ssl_cert=/etc/mysql/galera-cert.pem;socket.ssl_key=/etc/mysql/galera-key.pem"',
      "wsrep_cluster_name"     => "foo",
    }
  }
)
```

See http://www.mysqlperformanceblog.com/2013/05/03/percona-xtradb-cluster-for-mysql-and-encrypted-galera-replication/ for more info.