B #-: Monitor CLOSE-WAIT stalled connections (#755)

For vCenter monitor, already added hosts were added again, provoking a
new call to vcenter_connect. This call makes a new vSphere connection
and leaving the previous one hanging. After a timeout, vCenter tried to
close the connection by sending a TCP FIN. OS TCP stack cannot tell
the application (vcenter_monitor) to close, resulting on a CLOSE-WAIT
stalled connections (last forever). This in turn, exhausts the maximum
number of file descriptors opened allowed per process.

Signed-off-by: Ricardo Diaz <rdiaz@opennebula.io>
(cherry picked from commit 8de3e9c87215f5d70bdd80f938b03a038ef6a569)

src/im_mad/remotes/lib/vcenter_cluster.rb

@@ -751,6 +751,8 @@ class ClusterSet
 
     # Add a host by id, it access OpenNebula to get connection parameters
     def add(hid, conf)
+        return if @mutex.synchronize { @clusters.key?(hid) }
+
         begin
             cluster = Cluster.new(hid, @client)
             error   = ''