lint: disable import-error on external 3rd party modules

The new pylint runs in a unique isolated environment that doesn't have
access to any system installs.  This means it's unable to locate any
system installed 3rd party modules leading to import-error warnings.
Until we can figure out what approach we want to take here, disable
the warning on such modules.

BUG=chromium:866772
TEST=`cros lint` is clean again

Change-Id: I0337672cde81431f3aa418cff8074e0e072e0f71
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/1769193
Reviewed-by: LaMont Jones <lamontjones@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/sysmon/puppet_metrics.py b/scripts/sysmon/puppet_metrics.py
index 47614c0..ffd62f0 100644
--- a/scripts/sysmon/puppet_metrics.py
+++ b/scripts/sysmon/puppet_metrics.py
@@ -10,7 +10,7 @@
 
 import time
 
-import yaml
+import yaml  # pylint: disable=import-error
 
 from chromite.lib import cros_logging as logging
 from chromite.lib import metrics