tests.cluster: Add test for VerifyConfig

Check if VerifyConfig complains about orphan disks.
doc: Extend the design document for disks

Add more implementation info about the attach/detach operations and provide
examples for their invocation from the command line. Also, mention that detached
disks should be considered normal for a configuration and therefore, that we
remove the associated checks.
man: Update the gnt-instance man page

Add instructions on how to use the attach/detach options.