docs: fix links in the quickstart guide.

docs: fix links in the quickstart guide.

Fix installation and Traffic gen guide links in
http://artifacts.opnfv.org/vswitchperf/docs/userguides/quickstart.html
to
http://artifacts.opnfv.org/vswitchperf/docs/userguides/installation.html
and
http://artifacts.opnfv.org/vswitchperf/docs/userguides/trafficgen.html.
Also added where vloop_vnf can be downloaded from.

JIRA: VSPERF-186
Change-Id: I585367b1c717fdbb0b56b7f0312fe9c70f51594f
Signed-off-by: Maryam Tahhan <maryam.tahhan@intel.com>
Reviewed-by: Martin Klozik <martinx.klozik@intel.com>
Reviewed-by: Radek Zetik <radekx.zetik@intel.com>
Reviewed-by: Dino Simeon Madarang <dino.simeonx.madarang@intel.com>
(cherry picked from commit a844a969d48f2294e639350f5eca82bf8ef81623)