refactor: change the Add , Delete , Update ztp services . Add exception handling for external services and remove unnecessary device==null exception handling. Create an internal exception handling mechanism for all the uni failures.

Resolve "(UBI) Error handling in ZTP component"

See merge request !228

- Updated README.md file
- Fixed libyang version in Dockerfile
- Fixed libyang version in install_requirements.sh
- Added folder libyang to global gitignore
- Added generation of .gitignore file for libyang/build

Resolve "Fix libyang version in NBI component"

See merge request !230