Update gnmi_set.go to support leaf list node update operation and changes for json replace usning py_gnmicli.py(currently it is broken)

[tejaskumark]

I know commit is not as per standard, but still we can check for review.
You can update leaf list node using setting below flag,

Set command line argument flag "-leaf_list_node true"(default false) to set leaf list node values.
For Ex.

go run gnmi_set.go -insecure -username admin -password admin -target_name testap -target_addr 10.86.64.138:8080 -update /access-points/access-point[hostname=testap]/radios/radio[id=1]/config/allowed-channels:1,6 -leaf_list_node true

go run gnmi_set.go -insecure -username admin -password admin -target_name testap -target_addr 10.86.64.138:8080 -update /access-points/access-point[hostname=testap]/ssids/ssid[name=11-google]/config/supported-data-rates:RATE_48MB,RATE_36MB,RATE_54MB -leaf_list_node true

[codecov-io]

Codecov Report

Merging #47 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #47   +/-   ##
=======================================
  Coverage   52.79%   52.79%           
=======================================
  Files          10       10           
  Lines        1197     1197           
=======================================
  Hits          632      632           
  Misses        482      482           
  Partials       83       83

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