Add retry logic with a delay for vsphere connections

This commit adds retry logic and configurable delays to vsphere helper.
Without this change vmpooler instances that have large numbers of pools
can create enough connections in a short period of time to cause vcenter
issues.

vmpooler.yaml.example

@@ -232,6 +232,17 @@
 #     in an effort to maintain a more even distribution of load across compute resources.
 #     The migration_limit ensures that no more than n migrations will be evaluated at any one time
 #     and greatly reduces the possibilty of VMs ending up bunched together on a particular host.
+#
+#  - max_tries
+#    Set the max number of times a connection should retry in vsphere helper.
+#    This optional setting allows a user to dial in retry limits to
+#    suit your environment.
+#
+#  - retry_factor
+#    When retrying, each attempt sleeps for the try count * retry_factor.
+#    Increase this number to lengthen the delay between retry attempts.
+#    This is particularly useful for instances with a large number of pools
+#    to prevent a thundering herd when retrying connections.
 
 # Example: