execution hangs if not joined

[phemmer]

If you call CorosyncCommander#execute and you haven't joined the group yet, the app hangs.
It's hanging when trying to read a response, which makes sense as the message didn't go to the group, so there won't be any responses.

This should be solved such that we raise an exception if you try to execute something but haven't joined a group. I would have also expected the broadcast call to return an error, so see if it did and why we're not responding with an error as well.

2014-01-15 13:31:49 daemon/err(3) corosync[14572]:  [CPG   ] *** 0x7fe826387ee0 can't mcast to group  state:0, error:12