Overwrite provisioning playbook.yml in Vagrantfile

[joshavant]

This fixes a problem where, if vagrant provision is run more than once, the inline script from the Vagrantfile would append the existing contents the playbook.yml with another copy of the same script. After multiple copies have been redirected to playbook.yml, a YAML syntax error results. i.e. The following output will come from a second vagrant provision run:

==> default: ERROR! Syntax Error while loading YAML.
==> default: 
==> default: 
==> default: The error appears to have been in '/home/vagrant/ansible/roles-ubuntu/playbook.yml': line 6, column 1, but may
==> default: be elsewhere in the file depending on the exact syntax problem.
==> default: 
==> default: The offending line appears to be:
==> default: 
==> default:     - cis
==> default: ---
==> default: ^ here
The SSH command responded with a non-zero exit status. Vagrant
assumes that this means the command failed. The output for this command
should be in the log above. Please read the output to determine what
went wrong.

This PR changes the inline script so that playbook.yml will be overwritten each time vagrant provision is called.

[coveralls]

Coverage decreased (-0.5%) to 87.261% when pulling bd22b65 on joshavant:vagrantfile-provisioning-fix into c346162 on awailly:master.

[pchaigno]

@joshavant Thanks for the pull request. While you're at it, could you also change it in the README?

@awailly Was there a reason for appending the new content rather than creating a new file? (This is from the general installation steps we have since the beginning.)