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Add TAG_ADD_OPT option to cli.py and use it in gnt-instance. Modify
cli.GenericInstanceCreate() accordingly.
Signed-off-by: Apollon Oikonomopoulos <apollon@noc.grnet.gr>
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>