config: Remove checks for detached disks

It is not an error sign for a configuration to have detached disks, since they
can be handled from now on by the attach/detach operations.
burnin: Test if attach/detach works properly

The attach/detach test is merged with the add/remove test. The test is
the following: an extra disk is added in each instance and is given a
random name. Once it has been added successfully, we detach it and check
if the instance operates normally. Then, we pick instances randomly and
attach a detached disk to them, using an algorithm that matches a disk
to an instance based on its disk nodes. Then, we remove the disk.

Also, move the start/stop at the top of the tests, since the start/stop
functionality is erroneously used first by other functions (e.g. rename)
and then tested on its own.
tests.config: Add test for GetDiskInfoByName

Check if the GetDiskInfo and GetDiskInfoByName functions operate as
expected.
tests.cmdlib: Test index operations

Test if the GetIndexFromIdentifier and InsertItemToIndex functions work
properly.
tests.opcodes/client: Add test for constants

Also, test if the ConvertNicDiskModifications function operates as
expected.
tests.config: Test attach/detach wrappers

Test if the AttachInstanceDisk and DetachInstanceDisk wrappers update
the configuration as expected.
tests.cmdlib: Check if attach/detach checks work

Also, update the mocked config to create orphan disks for attach
purposes.
tests.cmdlib: Extend tests for _ApplyContainerMods

Test if _ApplyContainerMods works properly in attach/detach scenarios.
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.