Make hostname checks uniform between instance rename and add

Make hostname checks uniform between instance rename and add

Currently, we have instance rename doing extra checks on the host
name, to prevent accidental wrong renames; however, instance create
doesn't do these checks (issue 291), which (if DNS is misconfigured)
can lead to hard to diagnose errors.

This patch abstracts the name checking from LUInstanceRename into a
separate function, which is then reused in both instance rename and
instance create.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Bernardo Dal Seno <bdalseno@google.com>