CLOUDSTACK-6650: Reorder Cluster list in deployment planner to protect
GPU enabled hosts from non-GPU VM deployment.
Cluster reordering is based on the number of unique host tags in a cluster,
cluster with most number of unique host tags will put at the end of list.
Hosts with GPU capability will get tagged with implicit tags defined by
global config param 'implicit.host.tags' at the time os host discovery.

Also added FirstFitPlannerTest unit test file.