Inconsistency use of python command in AuTest

[masaori335]

Both of python and python3 are used in AuTest. On platforms which python command refers to v2.x, AuTest mixing python versions.

• e.g.
  python
  

  trafficserver/tests/gold_tests/body_factory/http_with_origin.test.py

  Lines 76 to 77 in b670607

  	trhead200.Processes.Default.Command = "python tcp_client.py 127.0.0.1 {0} {1} | grep -v '^Date: '| grep -v '^Server: ATS/'".\
  	format(ts.Variables.port, 'data/{0}_head_200.txt'.format(HOST))

  
  python3
  

  trafficserver/tests/gold_tests/tls/tls_0rtt_server.test.py

  Line 144 in b670607

  tr.Processes.Default.Command = 'python3 test-0rtt-s_client.py {0} {1} {2} {3}'.format(ts.Variables.ssl_port, 'h1', 'get', Test.RunDirectory)