Unify the instance creation code

Currently the AddInstance in gnt-instance and ImportInstance in
gnt-backup duplicate all of their code except the actual opcode creation
(the parameters to it). By moving this to cli.py (not optimal location,
but we don't have another one), we can use a single copy of the code,
and simply change based on the mode parameter the few different
opcode parameters.

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