implement selectTarget algorithm to promote host-internal clones
Merge branch 'master' into clone
implement the inner host target selection algorithm

we improve the vm clone process by encouraging more clones inside the same host. The metric Scarcity as we
named, is to measure the urgency a host needs a clone. The scarcity basically evaluates how many rounds of
clone process is required to complete the cloning process for all nodes on that host.

The targetSelector is implemented as Strategy pattern in FastCloneService, i.e., you can inject different
selection algorithms based on your needs.
Merge branch 'clone'
Merge branch 'master' of github.com:vmware-serengeti/serengeti-ws