#### Why I did it
To eliminate the need to write duplicate code in order to import a Python module from a source file.
#### How I did it
Add `general` module to sonic-py-common, which contains a `load_module_from_source()` function which supports both Python 2 and 3.
Call this new function in:
- sonic-ctrmgrd/tests/container_test.py
- sonic-ctrmgrd/tests/ctrmgr_tools_test.py
- sonic-host-services/tests/determine-reboot-cause_test.py
- sonic-host-services/tests/hostcfgd/hostcfgd_test.py
- sonic-host-services/tests/procdockerstatsd_test.py
- sonic-py-common/sonic_py_common/daemon_base.py
Fix a strange bug introduced by https://github.com/Azure/sonic-buildimage/pull/6832 which would only occur in environments with both Python 2 and Python 3 installed (e.g., the PMon container). Error messages such as the following would be seen:
```
ERR pmon#ledd[29]: Failed to load ledutil: module 'importlib' has no attribute 'machinery'
```
This is very odd, and it seems like the Python 2 version of importlib, which is basically just a stub, is taking precedence over the Python 3 version. I found that this occurs when calling `import importlib`. However, calling `import importlib.machinery` and `import importlib.util` causes the proper package to be referenced, and the `machinery` and `util` modules are loaded successfully. This is how it is specified in examples in the official documentation, however there is nothing mentioned regarding that it *should* be done this way or that `import importlib` is unreliable.
Also, since sonic-py-common is still used in environments with Python 2 installed we should maintain support for both Python 2 and 3 until we completely deprecate Python 2, so I have added this back in.
Migrate from using the `imp` module to using the `importlib` module. As of Python 3, the `imp` module has been deprecated in favor of the `importlib` module.
**- Why I did it**
Install all host services and their data files in package format rather than file-by-file
**- How I did it**
- Create sonic-host-services Python wheel package, currently including procdockerstatsd
- Also add the framework for unit tests by adding one simple procdockerstatsd test case
- Create sonic-host-services-data Debian package which is responsible for installing the related systemd unit files to control the services in the Python wheel. This package will also be responsible for installing any Jinja2 templates and other data files needed by the host services.
