Apps that are explicitly not to be platform_managed are being picked

Brief Description
-----------------
Apps that explicitly define platform_managed_app as 'no', 'false' or '0' are being picked up to be platform managed nonetheless.

Probably a misuse of common.utils.is_valid_boolstr at https://github.com/starlingx/config/blob/master/sysinv/sysinv/sysinv/sysinv/conductor/kube_app.py#L2583

Severity
--------
Minor: System/Feature is usable with minor issue

Steps to Reproduce
------------------
Write down the steps to reproduce the issue

Expected Behavior
------------------
App is not picked up to be platform managed on sysinv's periodic task

Actual Behavior
----------------
App is picked up to be platform managed on sysinv's periodic check

Reproducibility
---------------
5/5

Test Activity
-------------
Developer Testing

Workaround
----------
remove platform_managed_app from metadata.yaml