Only use slot for provider management

This commit reverts thread assignment for check_pool to a per-pool basis. Without this change a set of threads is shared in a manner that is not guaranteed to be thread safe.

lib/vmpooler/pool_manager.rb

@@ -601,7 +601,7 @@ module Vmpooler
 
       $providers[slot] ||= Vmpooler::VsphereHelper.new $config, $metrics
 
-      $threads[slot] = Thread.new do
+      $threads[pool['name']] = Thread.new do
         _check_pool(pool, $providers[slot])
       end
     rescue => err