overlord: rename all occurrence of cid to sid

Rename all cid to session ID to prevent confusion.  All client is now
identified as their mid(machine ID) and sid(session id).

BUG=none
TEST=run overlord and ghost client (both go and python version)

Change-Id: I51e25c30c3a0bdebc3fb6538b73c7ea0d9f9ace8
Reviewed-on: https://chromium-review.googlesource.com/291401
Commit-Ready: Wei-Ning Huang <wnhuang@chromium.org>
Tested-by: Wei-Ning Huang <wnhuang@chromium.org>
Reviewed-by: Hsu Wei-Cheng <mojahsu@chromium.org>
diff --git a/py/tools/ghost.py b/py/tools/ghost.py
index 2d4d240..ad4b5be 100755
--- a/py/tools/ghost.py
+++ b/py/tools/ghost.py
@@ -108,7 +108,7 @@
     self._mid = mid
     self._sock = None
     self._machine_id = self.GetMachineID()
-    self._client_id = sid if sid is not None else str(uuid.uuid4())
+    self._session_id = sid if sid is not None else str(uuid.uuid4())
     self._browser_id = bid
     self._properties = {}
     self._shell_command = command
@@ -616,7 +616,7 @@
         self._machine_id = self.GetMachineID()
         self.SendRequest('register',
                          {'mode': self._mode, 'mid': self._machine_id,
-                          'cid': self._client_id,
+                          'sid': self._session_id,
                           'properties': self._properties}, handler)
       except socket.error:
         pass
@@ -701,7 +701,7 @@
   def Start(self, lan_disc=False, rpc_server=False):
     logging.info('%s started', self.MODE_NAME[self._mode])
     logging.info('MID: %s', self._machine_id)
-    logging.info('CID: %s', self._client_id)
+    logging.info('SID: %s', self._session_id)
 
     # We don't care about child process's return code, not wait is needed.  This
     # is used to prevent zombie process from lingering in the system.