Add tagging option to gnt-instance create

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>