filter_scheduler: Select from a subset of hosts.

Adding a subset of best hosts to choose a host from, instead
of choosing the host with the highest weight. The subset size is
configurable. Setting the subset size to 1 will mimic the old
behavior.

DocImpact

Change-Id: Icc6f8a7ca7cf992d99b872a45e241f13fa6e1536
Fixes: bug 1098635
Merge "filter_scheduler: Select from a subset of hosts."