move-instance: Allow overriding instance parameters

When moving a single instance within the same cluster, the NIC
is not allowed to re-use an existing MAC address. To avoid this,
NIC parameters must be overridden. BE, HV, OS and NIC parameters
can be overridden after applying this patch.

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