Document and verify versioning of upgrade images

[jgallag88]

Upgrade images contain a VERSION field which is tied to the version of the delphix-virtualization package. Somewhere we should document the details of how this works, since this is made somewhat more complicated following #166. Some points to touch on include

• How the versions of installed packages (both Delphix packages and 3rd party packages) are kept consistent across the different VM artifacts and delphix-entire- packages created within the same build.
• Why this consistency matters for upgrade.
• How the version of delphix-virtualization is used when building delphix-entire- and the upgrade images.
• What happens if delphix-virtualization is not installed, (as with the internal-minimal variant, for example).

We should also check that all of the versions of the delphix-entire- packages match when we construct an upgrade image, as a verification that this design is working as intended.

[pzakha]

@jgallag88 I believe this bug is somewhat obsolete since we are not using the version of delphix-virtualization anymore as the version for delphix-entire. Is there anything in here that we still need to document?