Using SCM_WEB macro in documentation.

[acloudiator]

To validate and correct the macro in the documentation (wherever applicable).

Originally this was brought up Mauricio:

As a note: This works fine for now, but once do the first release this will cause problems because the documentation of that release would point to this the vagrant file in master.
Using the `SCM_WEB` macro is better.

Originally posted by @mauriciovasquezbernal in https://github.com/polycube-network/polycube/pull/83/files/460a55fe0040f2b352e8ef78a8a5ea7d73b1b97b