Slight change to the LU initialization code

This patch adds support for a separate LU.CheckArguments() method which
should do syntactic checks without holding locks and without polluting
the ExpandNames which is a lock-related function. See for example the
FIXME in LUSetInstanceParams.CheckPrereq.

The patch also removes the REQ_MASTER and IsCluster checks since with
the current architecture, inside LU code we are always in a valid
cluster and we are always running on the master node.

Reviewed-by: ultrotter