Fix: Ensure Vagrant default provisioning mode is "once"

[Berbe]

Considering:

• Default value of python-vagrant for provision is None
• python-vagrant documentation states the boolean is optional, defaulting to Vagrant default behaviour (which is to run provisioners once on creation)

The Molecule's Vagrant plugin currently override the default value, defaulting to False, and only settable to True (None is rejected, not being a boolean). This PR suggests a change so the default value of the Molecule plugin is aligned with python-vagrant's one, and eventually with Vagrant's behaviour.