VMware: Create VMwareImage object for image metadata

This change pulls the nested function _get_image_properties() out of
spawn() and makes it a factory method for the new VMwareImage object.

Linked clone logic is moved from a separate function in vmops.py to
the new get_image_properties() function.

vmware_images.get_vmdk_size_and_properties() is no longer used, and is
removed.

Tests have been updated and/or moved as appropriate. No tests have
been removed.

This change has been split out of
https://review.openstack.org/#/c/87002/, which was written by Shawn
Hartsock.

partial blueprint vmware-spawn-refactor

Co-authored-by: Shawn Hartsock <hartsocks@vmware.com>
Change-Id: I580f173da798318d2675c7c70bbdd19b266259f4
Merge "VMware: Create VMwareImage object for image metadata"