Readd node: allow changed primary IP

The primary IP is not used for DRBD disks, only for the job queue and
internal commands. As such, it's trivial (with a quiet job queue) to
re-ip the node.

The patch just relaxes the checks in LUAddNode, and manual testing shows
that live instances on that node keep working, and that cluster verify
is happy afterwards.

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