LUInstanceRename: Fail if renamed hostname mismatch

There's a problem if you run gnt-instance rename with a non FQDN and the
renamed LU tries to resolve the hostname to make it FQDN. It could be
that this resolved hostname was just a CNAME to another name which leads
to wrongly renamed instances name.

Given a name of "foobar" is a CNAME to "moohost" now if you gonna rewrite an
instance named "foobar1" to "foobar" it will pickup the host "moohost".
This is not obvious behaviour. If we detect this case we abort with a
prereq error. The user then however can continue with a FQDN and
--no-name-check if he intends to force the operation.

Signed-off-by: René Nussbaumer <rn@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>