Add 2 new variables to the OS scripts environment

Add INSTANCE_PRIMARY_NODE and INSTANCE_SECONDARY_NODES. These new
values are useful for OS scripts that needs to know the nodes where
the instance lives.. or has lived.

Signed-off-by: Iustin Pop <iustin@google.com>
[iustin@google.com: fixed small issue with SECONDARY_NODES]
Reviewed-by: Iustin Pop <iustin@google.com>