mark oslo.vmware as optional dependency

mark oslo.vmware as optional dependency

Driver-specific requirements are not hard requirements, since the
choice of drivers is up to the operator. The oslo.vmware module is
herein moved out of requirements.txt and instead listed as an optional
dependency using the extras functionality in setup.cfg.

A check is added to the vmdk driver to gracefully handle import errors
if the vmdk driver is used and oslo.vmware is not found.

Change-Id: I9b00edc38f0700304a1a164f0679a734f8701ebe
Closes-Bug: #1475739