vagrant: Enable silent-rules for configure.

In the majority of cases, developers debugging their code using vagrant
will be more interested in compiler errors/warnings than the exact
invocation of the compiler. By enabling silent-rules, the verbosity of
compilation is lowered and it is easier to identify these pieces of
information.

Signed-off-by: Joe Stringer <joe@ovn.org>
Acked-by: Andy Zhou <azhou@ovn.org>
Acked-by: Ryan Moats <rmoats@us.ibm.com>

Vagrantfile

@@ -20,7 +20,7 @@ SCRIPT
 $configure_ovs = <<SCRIPT
 mkdir -p ~/build
 cd ~/build
-/vagrant/configure --with-linux=/lib/modules/`uname -r`/build
+/vagrant/configure --with-linux=/lib/modules/`uname -r`/build --enable-silent-rules
 SCRIPT
 
 $build_ovs = <<SCRIPT