Modify yang modules such that it can be validated using pyang

Pyang is a yang modules validator which is used by the yang modules community. So to publish the yang modules in the yang modules Github it should be able to verify the yang modules using pyang. Right now it is giving some errors, the link to which is provided below.

https://logs.opendaylight.org/releng/vex-yul-odl-jenkins-1/distribution-merge-managed-magnesium/958/opendaylight-models/ovsdb/errors.txt.gz