Revert 1154253 causes XenServer image compat issue

In 1154253 we:

  Drop gzip flag from tar command for OVF archives

While support more OVF file formats is a worthy goal the approach
in 1154253 is going to cause serious issues for existing
Nova XenServer users who are already zipping their OVS images.

I would also point out that our utils.py xenserver plugins still
zips tarballs we create (see create_tarball) so the new code
causes internal inconsistencies as well.

This should fix SmokeStack.

Fixes LP Bug #1155783.

Change-Id: Iff6db314c0dec6df2e48d98db2a547f6624b9d9a
Merge "Revert 1154253 causes XenServer image compat issue"