docs: update 2n-skx job_specs for report_iterative

docs: update 2n-skx job_specs for report_iterative

Changes in vpp-2n-skx.md:
- all nfv_density moved to separate file vpp-2n-skx-nfv-density.md
- removed intel-x710: dot1q
- removed drv_vfio_pci: vts, dot1q

- Rebased on https://gerrit.fd.io/r/c/csit/+/27495
- *.md files moved to docs/job_specs/report_iterative/2n-skx/ and
  renamed

Change-Id: Iff16f6673ea3d829c44fbd04d892abb845fa4c1e
Signed-off-by: Maciek Konstantynowicz <mkonstan@cisco.com>
(cherry picked from commit 6fa263815306b43ab1813360f0123e8a245b4b56)