refactoring the Galera bootstrap

Tested in RHEL, not on Debian yet.

Remaining issues:

- gcomm:// cluster nodes should be generated using shared ressource to
  avoid the following case:
    gcomm:// is initialized with all the node of the cluster,
    none of them are ready,
    the server is bootstrapped and we have to reboot it.
    → In this case, the serve reboot will fails.
    That's not the case if gcomm:// comes only with working nodes.

closes #151
import galera_is_bootstrapped() definition
exec db_sync script when galera is ready

We use the new galera_is_bootstrapped() function to call db_sync
script only when Galera is ready. This reduce a lot the puppet run
time in such case.
clean up

We use the same mechanism on Debian and RedHat now.