vagrant: Add flag to ensure install is completed at least once

The provisioning script used to check for the existence of the
`/opt/edx/node_modules` folder to decide if the installation has already
been completed. However, since additional instructions were executed
by the installer afterwards, any installation error (eg. download
timeouts on low quality connections) happening between the creation of
that folder and the end of the installation wouldn't be recovered from
on the next vagrant instance boot.
Merge pull request #634 from antoviaque/vagrant-provisioning-resume

vagrant: Add flag to ensure install is completed at least once