Goal:
2nd stage of validation in VES Collector is dedicated to check the validity of events transferred as payload of VES stndDefined messages. It is expected to improve the troubleshooting of network functions, which bring standards-defined notifications to ONAP.

1. VES Collector is enhanced with the 2nd stage of JSON documents validation
   1. In case a configurable parameter (controlling 2nd stage of validation) is enabled
2. VES collector executes the 2nd stage of validation:
   1. If the parameter mentioned before allows for that validation action
   2. If the stndDefinedFields.schemaReference parameter is present
   3. If the stndDefinedFields.schemaReference points to a valid schema stored in associated repository
3. Following are characteristics of 2nd stage of validation
   1. 2nd stage validation - covers contents of stndDefinedFields.data property
   2. 2nd stage validation - is executed with cached schemas, to reduce impact on validation performance
4. Error cases are covered (at least those):
   1. stndDefinedFields.data empty
   2. schema reffered by stndDefinedFields.schemaReference doesn`t exist of is inaccessbile
5. Absence of stndDefinedFields.schemaReference is a normal case (parameter is optional).