Introduce a master playbook

- add git.yml playbook to install from homebrew AND set some global settings
- software playbooks will have configuration variables (e.g. version,...)
- master playbook can/should be used to define this variables
- update virtualbox to version 4.3.20
- update vagrant to version: 1.7.1
- include a dummy ansible localhost/workstation inventory for
  ansible-playbook debugging purpose