Proposal for using var_files and conditional imports #9

I've done some changes using var_files and conditional imports to make the roles more dynamic. Added a test Vagrantfile and playbook so we can make some tests more easily with the roles in this repo.

In this scenario we only need to change variables in vars/common.yml. The "web_server" var in the playbook can also be moved to this file. The roles import is conditional.

Added a role called "app" for tasks that should be executed after the main provisioning is finished. My suggestion is to add the timezone configuration from #7 into this task file, using a conditional for the php.ini change specific for Apache - `when: web_server == 'apache'`

With the conditional vars and imports we can deal more easily with different operating systems too.

Still need to figure out the best way to deal with Apache's different versions.