gnt-cluster modify: limit storage type consistency check

This fixes the currently broken Q&A. The consistency check
for storage types is now only run when the 'enabled_storage_types'
are actually changed by 'gnt-cluster modify'.

Signed-off-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>