Fix conf file open mode when "tempest init"

This commit fixes the conf file open mode when "tempest init" command
runs. The original open mode "w" truncates its file. We should use 'a'
for updating a file instead.

Change-Id: I033454950cf58615e212d8c9fdf96765cf45c454
diff --git a/tempest/cmd/init.py b/tempest/cmd/init.py
index d8d9d7c..c2b3ce1 100644
--- a/tempest/cmd/init.py
+++ b/tempest/cmd/init.py
@@ -98,7 +98,7 @@
     def update_local_conf(self, conf_path, lock_dir, log_dir):
         config_parse = moves.configparser.SafeConfigParser()
         config_parse.optionxform = str
-        with open(conf_path, 'w+') as conf_file:
+        with open(conf_path, 'a+') as conf_file:
             config_parse.readfp(conf_file)
             # Set local lock_dir in tempest conf
             if not config_parse.has_section('oslo_concurrency'):