core: UpdateStoragePoolCommand refactoring.

UpdateStoragePoolCommand's CDA was refactored to fit the CDA style in
the system. as a side effect this makes the code a bit more efficient
since once a validation fails within the CDA it makes the function
return and not continue to the next validation.

Change-Id: Icd0c28eb86af753a7e48e845da23fc5feca2019a
Signed-off-by: Amit Aviram <aaviram@redhat.com>