Waiting for a port before trying to connect in buildbot scripts

Currently, the buildbot scripts that check SST in Galera regularly give us false red cells in the test results tables with non-existent failures due to the fact that the client (mysql) tries to join the cluster before the next server has passed to the listening state on the specified port number. And in the case of SST, rsync also calls kill on rsync, resulting in an unhealthy cluster, since SST crashes on some nodes when rsync is asynchronously killed prematurely. Therefore, we need to add a function to the buildbot scripts for Galera to wait for the server to enter the listening state on a given port number before trying to connect and remove kill on rsync from the connection attempt loop.