Stop using subprocess for testr init in tempest init

This commit switches the call to testr init from using subprocess to
testr's python run_argv interface. (which is essentially just calling
it via subprocess) The issue with using subprocess directly is that it
assumes testr is in the PATH you're running from. But, in the case of
using a venv this isn't always the case.

Change-Id: I2bab70b0e9fb2ccdc340f48d95340ff7ccdc1364
Closes-Bug: #1633182

tempest/cmd/init.py

@@ -14,13 +14,13 @@
 
 import os
 import shutil
-import subprocess
 import sys
 
 from cliff import command
 from oslo_config import generator
 from oslo_log import log as logging
 from six import moves
+from testrepository import commands
 
 from tempest.cmd import workspace
 
@@ -167,7 +167,8 @@ class TempestInit(command.Command):
         self.generate_testr_conf(local_dir)
         # setup local testr working dir
         if not os.path.isdir(testr_dir):
-            subprocess.call(['testr', 'init'], cwd=local_dir)
+            commands.run_argv(['testr', 'init', '-d', local_dir], sys.stdin,
+                              sys.stdout, sys.stderr)
 
     def take_action(self, parsed_args):
         workspace_manager = workspace.WorkspaceManager(