Vagrant: Failed if user neither defines DLV_PORT nor run make before vagrant up

[elfadel]

If the user doesn’t define DLV_RPORT variable during vagrant up and doesn't execute make beforehand, the vagrant up command failed with the following message:

There are errors in the configuration of this machine. Please fix
    the following errors and try again:
    
    vm:
    * Forwarded port definitions require a "host" and "guest" value
    * Ports to forward must be 1 to 65535

This PR prevents this failure by setting up default ports if the user doesn't specify anyone.