Fix un-assignment local variable 'returncode' error

This commit fixes an error "local variable 'returncode' referenced
before assignment" when we run 'tempest run' with having a
testrepository.

Change-Id: I99425de8481db4bddbe06b715bcceaaadaa1a797
diff --git a/tempest/cmd/run.py b/tempest/cmd/run.py
index e78f6b0..26bd418 100644
--- a/tempest/cmd/run.py
+++ b/tempest/cmd/run.py
@@ -70,6 +70,7 @@
 
     def take_action(self, parsed_args):
         self._set_env()
+        returncode = 0
         # Local execution mode
         if os.path.isfile('.testr.conf'):
             # If you're running in local execution mode and there is not a
@@ -77,8 +78,8 @@
             if not os.path.isdir('.testrepository'):
                 returncode = run_argv(['testr', 'init'], sys.stdin, sys.stdout,
                                       sys.stderr)
-            if returncode:
-                sys.exit(returncode)
+                if returncode:
+                    sys.exit(returncode)
         else:
             print("No .testr.conf file was found for local execution")
             sys.exit(2)