VMware: use datastore classes get_allowed_datastores/_sub_folder

Convert the above-mentioned functions to use the datastore utility
classes.

partial blueprint vmware-spawn-refactor
Change-Id: I20c37367c8dca1c8dc20a308ce2cea5229097701
Revert "Add missing image to instance booted from volume"

This reverts commit c3191cf0ba5ad3dc2df8da2a2bf5c9d270fde9d9.

The change needs to be thought out some more, i.e. why isn't
compute_api.get_all just loading up the image_ref value on the instance
object rather than calling a helper method from the servers extension to
the instance object which just calls back to compute_api, seems very
roundabout, plus isn't it a v3 API issue also?

Conflicts:
	nova/api/openstack/compute/servers.py

Due to: 826aed0 Use oslo.i18n

Change-Id: I6abbfdfa786c3d98065c969e7f9d7d5830caf7e8
Closes-Bug: #1343689
Merge "VMware: use datastore classes get_allowed_datastores/_sub_folder"
Merge "Revert "Add missing image to instance booted from volume""