[1chb1n, r=thedac] Update mysql vs pxc inheritance and drop split-msg in sparse bundles
Precise needs to use the mysql charm which has different charm config options
than percona-cluster.
Non-Precise needs to use the percona-cluster charm which has different charm
config options than mysql proper.
Also simplify sparse bundles for single amqp only and save split-msg to a new
dev bundle for later use if necessary.