improve the Vagrant VMs

This bumps the VM memory and corrects forwarded ports when they're used
already.

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
Merge pull request #3557 from unclejack/improve_vagrantfile

improve the Vagrant VMs: bump RAM to 512 MB & correct forwarded ports