fix package name for RHEL distros:

* `Percona-XtraDB-Cluster-55` vs. `percona-xtradb-cluster-55`
update `cluster_spec` to match centos package name
add contribution from @mzdrale to the README
fix version toggle, install proper cluster package
fix XtraDB Cluster dependencies

* includes fixes from #211 - thanks @Sauraus!
README link updates

[ci skip]
add contribution from @Sauraus to the README
Merge pull request #220 from phlipper/mzdrale-rhel-xtradb-cluster

Attribute fixes for xtradb-cluster package