Updated microbosh.rb to allow update if the deployment exists, or create a new one if it does not exist.

Here is the new syntax for bosh micro deploy:

micro deploy [<stemcell>] [--update] [--update-if-exists]
    Deploy a micro BOSH instance to the currently selected deployment
    --update                              update existing instance
    --update-if-exists                    create new or update existing instance


When I ran bosh-bootstrap deploy on an existing instance it fails asking if I meant to update. The newer syntax allows for --update-if-exists in addition to --update and it works very well. I tested it.