Allow use of /etc/os-release file

The top-level Makefile in VPP already uses the file /etc/os-release to
recognize which OS the build is being performed on.
The scripts for vagrant images instead were still using different files
for different distros and adding a new distro to be supported causes
issues with the extra-logic to deal with it.
This patch aligns the vagrant scripts to use the same approach followed
by the top-level Makefile and simplifies the steps to identify / update
the OS and build / run VPP.

Change-Id: I539f4a7c27b3fef70ed1c12b0276049bc47dc289
Signed-off-by: Marco Varlese <marco.varlese@suse.com>