support configuration of number of virtual nodes

I recently played with consistent hash function implementation and produced various performance statistics.
It shows that number of virtual nodes (configuration parameter I98 in https://docspace.corp.redhat.com/docs/DOC-69249) has substantial effect on performance. (it makes distribution of data among nodes in the cluster much more even and thus avoids overstressing of one particular node, that would then hinder performance of the whole cluster)

If the next EDG6 Alpha build will take configuration from infinispan subsystem, we'll need to have this configurable, otherwise we won't be able to execute some very important performance comparisons.

it applies to the level of subsystem.cache-container.distributed-cache

so I propose addition of configuration parameter:
subsystem.cache-container.distributed-cache.numVirtualNodes that would have the same effect as I98 infinispan.configuration.clustering.hash.numVirtualNodes