Cleanup LU.ExpandNames versus CheckArguments

When LogicalUnit.CheckArguments was introduced, not all code dealing
with static argument checking was moved to it; many of these checks were
left in ExpandNames. With time, most of them migrated, and this patch
does the final cleanups.

The patch is straightforward, with the exception of LURebootInstance,
where an old style ParameterError exception is converted to the new
OpPrereqError with ECODE_INVAL.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>