VMware: Add utility method to retrieve remote objects

Added a utility method that can retrieve the properties of objects
that are within a parent object in the vCenter server. With
existing utility methods, this is not possible without making multiple
api calls.

This patch partially re-adds the changes reverted by change-id
Ia3a6e3f502282d44473d4908f68d71a97167e85f.

Change-Id: I9a461b2cdde906d4f25af513b4c0410b35e82209
VMware: fix list_instances for multi-node driver

VMwareVCDriver should only list instances in the nodes managed by
it. Currently, it uses the an implementation that lists instances
in the vCenter server inventory even if they are not in the nodes
managed by the driver.

Closes-bug: #1272286
Change-Id: I56c81a759eacc8c595e97ac5ca372834b675ebff
Merge "VMware: Add utility method to retrieve remote objects"
Merge "VMware: fix list_instances for multi-node driver"