[sensu-percona] add initial percona cluster check

* Adds a sensu percona-cluster-size check for xtradb nodes
* Renamed compute[0] ansible host name to "db_arbiter"
* Removed start.yml tasks and moved mysql start task to percona.yml
* Makes adding root's .my.cnf defaults file DRYer
* Adds sensu sudo entry to sudoers file for the percona-cluster-size plugin