Allow updates to an existing microBOSH installation

Here is the new syntax for bosh micro deploy:

micro deploy [] [--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. 

Passed test at: https://travis-ci.org/lookitup4me/bosh-bootstrap/builds/13140446