Fix stability test issue

Fix stability test issue

Failures in stability tests are observed on attempts to get distribution status from SDC backend. These errors are caused by timeouts when reading from the DB (5 sec) and also the reads exceeding the tombstone threshold (100,000).
The fix implemented in this change is two fold:
1. Stop writing null values in empty columns in DB writes (a tombstone is created for each null column value)
2. Change the query to the DB to remove the unnecessary filtering which greatly increases the time taken for the read.

Failures in stability tests are also observed due to failures in parsing YAML files from VSP. The files are parsed using snakeYAML. The class used is not thread safe however it is possible for it to be invoked in multiple threads. This is most likely the cause of the parsing failures
The fix implemented here includes changes to ensure an instance of the Yaml class is not used across multiple threads

Signed-off-by: MichaelMorris <michael.morris@est.tech>
Issue-ID: SDC-3773
Change-Id: I4067ff0032bb67ab114db2f05accce5d7b42d188