Tolerate unknown host DCs more gracefully

For load balancing purposes, hosts are assumed to be in
the local datacenter now if the actual datacenter is unknown.

However, this is still a slight problem with contact points
in a remote DC when using DCAwareRoundRobinPolicy. See
https://datastax-oss.atlassian.net/browse/JAVA-138 for
more details.

Fixes #5