VMware: spawn refactor - DatastorePath class

Part of spawn refactor work to consolidate a lot of the datastore path
manipulation code. The DatastorePath class is a simpler version of a
class of the same name in earlier versions of
https://review.openstack.org/#/c/87002, and is intentionally limited in
the ways that DatastorePath objects can be constructed. No attempts are
made to ensure that the datastore, directories and files referenced in
these objects are valid on the remote system.

partial blueprint vmware-spawn-refactor

Change-Id: Id65d0308d32835048229e7a3d98b84af2b13583c
Merge "VMware: spawn refactor - DatastorePath class"