CLOUDSTACK-4435 [VMWARE]System VM's are failed to start with Nexus enabled Zone

Since introducing pool of session contexts we no more have a dedicated context for each VMware hypervisor host.
Hence vsm credentials stored in session context cannot be retrieved always correctly. Fix is to register the vsm credentials after fetching context and the context gets recycled after use.

Signed-off-by: Sateesh Chodapuneedi <sateesh@apache.org>