Remove checks wrt IDISK_PARAMS from OpCode level

Change the "--disks" option validation, to just check the format
of the dict and do not check whether the keys are included in the
IDISK_PARAMS constant at OpCode level. This allows the passing of
arbitrary parameters at the CLI, which will then be logically
checked at LU level (inside CheckArguments).

Currently, the check wrt to IDISK_PARAMS already happens in all
related LUs (InstanceSetParams, InstanceCreate, InstanceRecreateDisks),
with the use of ForceDictType(), so there is no need for additional
checking code.

Signed-off-by: Constantinos Venetsanopoulos <cven@grnet.gr>
Signed-off-by: Iustin Pop <iustin@google.com>
[iustin@google.com: fixed a unittest that needed adjustement]
Reviewed-by: Iustin Pop <iustin@google.com>