Cluster verify: gather node/instance list in CheckPrereq

This commit introduces no behavior changes, and is only a minor refactoring
that aids with a cleaner division of future LUClusterVerify work. The
change consists in:

  - substitute the {node,instance}{list,info} structures previously created
    in Exec() by member variables created in CheckPrereq; and

  - mechanically convert all references to the old variables to the new
    member variables.

Creating both self.all_{node,inst}_info and self.my_{node,inst}_info, both
with the same contents at the moment, is not capricious. We've now made
Exec use the my_* variables pervasively; in future commits, we'll break the
assumption that all nodes and instances are listed there, and it'll become
clear when the all_* variables have to be substituted instead.

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