support "show ntp" with mgmt vrf based on linux os version

[bsun-sudo]

- What I did
When mgmt vrf is configured, "show ntp" checks Linux version to determine if to use

- cgexec -g l3mdev:mgmt ntpq -p -n --> for 4.9.0
- ip vrf exec mgmt ntpq -p -n  ---> for 4.9.0+

- How I did it
changes in show/main.py to get linux version from os.uname() and then compare with 4.9.0

- How to verify it

on 4.19.0
//////////////////////
root@sonic:/usr/lib/python2.7/dist-packages/show# show ntp
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
*10.11.0.1       10.14.2.1        3 u    -   64    1    0.220  484.401   0.074
 10.11.0.2       10.14.2.1        3 u    1   64    1    0.258  481.733   0.206
root@sonic:/usr/lib/python2.7/dist-packages/show# uname -r
4.19.0-6-amd64
root@sonic:/usr/lib/python2.7/dist-packages/show#

on 4.9.0
////////////
root@sonic:/usr/lib/python2.7/dist-packages/show# show ntp
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
 10.11.0.1       10.14.2.1        3 u    1   64    1    0.213   -0.423   0.069
 10.11.0.2       10.14.2.1        3 u    2   64    1    0.331   -3.644   0.433
root@sonic:/usr/lib/python2.7/dist-packages/show#
root@sonic:/usr/lib/python2.7/dist-packages/show#
root@sonic:/usr/lib/python2.7/dist-packages/show# uname -r
4.9.0-11-2-amd64

- Previous command output (if the output of a command-line utility has changed)

- New command output (if the output of a command-line utility has changed)