add_garageadmin_account

### Changelog
* Created role to setup garageadmin account dealing with issue #3
* creates a new user account for garageadmin
* sets the password
* creates necessary directories
* assigns Admin privileges to account
* referenced in [this trello card](https://trello.com/c/QUyJSFTg/441-automate-creation-of-garageadmin-account-in-osx-bootstrap)

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I just tested this branch locally, and ansible is prompting for the admin password with every run even if the garageadmin account exists.  can we configure it to only ask for the password if it needs to create the user?