[autotest] Fix a bug in is_in_container check

BUG=chromium:705811
TEST=None

Change-Id: I46c3080e0ac2646b1594969f17fdfad98e0acafd
Reviewed-on: https://chromium-review.googlesource.com/486244
Commit-Queue: Dan Shi <dshi@google.com>
Tested-by: Dan Shi <dshi@google.com>
Trybot-Ready: Dan Shi <dshi@google.com>
Reviewed-by: Shuqian Zhao <shuqianz@chromium.org>
diff --git a/client/common_lib/utils.py b/client/common_lib/utils.py
index 92658c7..c61c7e0 100644
--- a/client/common_lib/utils.py
+++ b/client/common_lib/utils.py
@@ -2484,7 +2484,7 @@
         return True
 
     # Check "container" environment variable for lxd/lxc containers.
-    if os.environ['container'] == 'lxc':
+    if os.environ.get('container') == 'lxc':
         return True
 
     return False