An instance will now create a cluster only after contacting all instances

Before it was so that an instance would create a cluster if, during discovery,
 it would contact at least half of the instances configured as initial_hosts
 and it was the one with the numerically smallest instance id among them.
 This lead to problems where instances would go ahead and create different
 clusters because of startup order. In particular, an instance could
 respond to two other instances with numerically lower instance ids
 but those two would not manage to contact each other. That would
 lead them to each forming a cluster on its own and the remaining
 instance joining one of them, in random.
 This commit changes so that an instance no longer waits for
 quorum number of instances but all of them, as configured in
 initial_hosts. This ensures that the instance forming the cluster
 will have seen all that is possible to see and all other instances
 will join that cluster.
Adds description string to cluster test DSL verification method

ClusterMockTest.verifyConfiguration() can be called anywhere during
 a cluster test and is also called automatically at certain
 operations. This can lead to confusion in case of failures, as
 it's not obvious which call of the ones made during a test failed.
 Adding a description helps tell them apart.