LU.CheckPrereq: do not require implementation

Currently, the base class LogicalUnit's CheckPrereq will raise
NotImplementedError, which means that the child LUs have to implement
it. However, many LUs don't actually have a need for this function
(hence the many "pass" statements as the only body).

By changing the base class behaviour, we can simplify many LUs.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>