VMware: nova-compute crashes if VC not available

The ESX driver gets a connection to vCenter Server on startup.
Currently, if vCenter Server is not available, nova-compute crashes.
This fix adds slow retry when nova-compute starts before vCenter
Server is available.  The errors are logged in case the issue is
a mis-confiugration of vCenter Server

Fixes: bug #1192016

Change-Id: I0801ab4a4e4bf32b8119e131a45a2fdb7884a4fd
Merge "VMware: nova-compute crashes if VC not available"