pylint: Fix invalid-envvar-default lint error

The default argument of os.getenv() should be string type.

BUG=b:158811392
TEST=make lint && make test

Change-Id: I6bad984a6f082addba9354e2ec415c43c4e3e3b0
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/factory/+/2255987
Reviewed-by: Yilin Yang (kerker) <kerker@chromium.org>
Tested-by: Fei Shao <fshao@chromium.org>
Commit-Queue: Fei Shao <fshao@chromium.org>
diff --git a/py/tools/ghost.py b/py/tools/ghost.py
index 390aaf2..079387a 100755
--- a/py/tools/ghost.py
+++ b/py/tools/ghost.py
@@ -40,11 +40,11 @@
 from cros.factory.utils import process_utils
 
 
-_GHOST_RPC_PORT = int(os.getenv('GHOST_RPC_PORT', 4499))
+_GHOST_RPC_PORT = int(os.getenv('GHOST_RPC_PORT', '4499'))
 
-_OVERLORD_PORT = int(os.getenv('OVERLORD_PORT', 4455))
-_OVERLORD_LAN_DISCOVERY_PORT = int(os.getenv('OVERLORD_LD_PORT', 4456))
-_OVERLORD_HTTP_PORT = int(os.getenv('OVERLORD_HTTP_PORT', 9000))
+_OVERLORD_PORT = int(os.getenv('OVERLORD_PORT', '4455'))
+_OVERLORD_LAN_DISCOVERY_PORT = int(os.getenv('OVERLORD_LD_PORT', '4456'))
+_OVERLORD_HTTP_PORT = int(os.getenv('OVERLORD_HTTP_PORT', '9000'))
 
 _BUFSIZE = 8192
 _RETRY_INTERVAL = 2