implements a new feature: "BGP Allow list."
This feature allows us to control which IP prefixes are going to be advertised via ebgp from the routes received from EBGP neighbors.
As part of consolidating all common Python-based functionality into the new sonic-py-common package, this pull request:
1. Redirects all Python applications/scripts in sonic-buildimage repo which previously imported sonic_device_util or sonic_daemon_base to instead import sonic-py-common, which was added to the 201911 branch in https://github.com/Azure/sonic-buildimage/pull/5063
2. Replaces all calls to `sonic_device_util.get_platform_info()` to instead call `sonic_py_common.get_platform()` and removes any calls to `sonic_device_util.get_machine_info()` which are no longer necessary (i.e., those which were only used to pass the results to `sonic_device_util.get_platform_info()`.
3. Removes unused imports to the now-deprecated sonic-daemon-base package and sonic_device_util.py module
This is a step toward resolving https://github.com/Azure/sonic-buildimage/issues/4999
