Implement OSHandlerSLES

* Support SLES11 SP4, SLES12 SP5, SLES 15 /SP1/2
* Identify abstract, private and public functions for OSHandler
* Introduce _pm* functions for OS specific package manager calls
* Create general functions blank_migration, capsule_migration,
  classic_migration and clean_and_update
* Add python-rpm dependency for SLES
* Reimplement _check_package_version as function of OSHandler
* Reimplement check_package_installed as function of OSHandler
* Add function to express mandatory Python module imports
* Add output suppression for exec_command, exec_failok, and exec_failexit