add galera pkg version to avoid version conflicts

Percona's xtradb 5.5 packages automatically install the latest percona-xtradb-cluster-galera package (currently: percona-xtradb-cluster-galera-3.x). We need to use percona-xtradb-cluster-galera-2.x with percona-xtradb-cluster-server-5.5.

3.x creates the following error when attempting to start percona-xtradb-cluster-server-5.5:
<pre>*** glibc detected *** /usr/sbin/mysqld: double free or corruption (!prev): 0x000000000162aa50 ***</pre>