VMware: Convert vmops to use instance as an object

This is mostly mechanical, but it does expose what appears to be a bug
in the interaction between object.Instance's config_drive field and
configdrive.required_by(). object.Instance defines config_drive to be
a String, but configdrive.required_by() expects it to be a Boolean. In
assigning a boolean value to config_drive, it is cast to a String,
which evaluates as True when cast back to a Boolean. We therefore
update the tests which use config_drive to set either a String value,
or None, which is what other code does.

Note that Nova does not pass an Instance object to all driver methods,
so not all uses can be updated.

Change-Id: Ifc9ce5d96da30abfa09b608a9c8615ede1485a50
Merge "VMware: Convert vmops to use instance as an object"