Make the vagrant host port configurable

If port 3000 is already being used then allow the user to override it
during provisioning using the POPIT_PORT environment variable.
Optionally allow NFS in vagrant configuration

This speeds up shared folder access, but does require the user to enter
their password, so is off by default.
Allow vagrant memory to be configured

Rather than having to delve into the config file, allow the amount of
memory to be configured through an environment variable.