Stop streaming from server when log file removed

The current streaming code will never terminate the streaming (unless
the client initiates the termination). Let's change it so that we stop
streaming when the log file is removed (job finishes). Also, no need to
check for log truncation since that doesn't happen for these logs.

Change-Id: I73f04ffe998e88b89aa660be05e645d3defb87cc
diff --git a/zuul/lib/log_streamer.py b/zuul/lib/log_streamer.py
index 59d5240..6695723 100644
--- a/zuul/lib/log_streamer.py
+++ b/zuul/lib/log_streamer.py
@@ -144,6 +144,11 @@
                 else:
                     break
 
+            # See if the file has been removed, meaning we should stop
+            # streaming it.
+            if not os.path.exists(log.path):
+                return False
+
             # At this point, we are waiting for more data to be written
             time.sleep(0.5)
 
@@ -159,16 +164,6 @@
                 if not ret:
                     return False
 
-            # See if the file has been truncated
-            try:
-                st = os.stat(log.path)
-                if (st.st_ino != log.stat.st_ino or
-                    st.st_size < log.size):
-                    return True
-            except Exception:
-                return True
-            log.size = st.st_size
-
 
 class CustomForkingTCPServer(ss.ForkingTCPServer):
     '''