test: fix 'etcd_helper'

Added default value to 'os.getenv()', so tests won't fail if 'ETCD_PATH' is not set.

test/etcd_helper.py

@@ -17,7 +17,7 @@ def __init__(self, host, port, workdir):
 
     def start(self):
         popen = subprocess.Popen(
-            [os.getenv("ETCD_PATH") + "etcd"],
+            [os.getenv("ETCD_PATH", default="") + "etcd"],
             env={"ETCD_LISTEN_CLIENT_URLS": self.endpoint,
                  "ETCD_ADVERTISE_CLIENT_URLS": self.endpoint,
                  "PATH": os.getenv("PATH")},