Update system_settings.py
Update system_settings.py

I figured since [Sal's 'Settings' wiki page](https://github.com/salopensource/sal/wiki/Settings) states the following...
```
There are defaults set in sal/system_settings.py, but they can be overridden if you choose
```
... that it might be a good idea to enumerate most every toggle-able feature within the `server_settings.py` file. That said... 

This PR suggests the following changes to `sal/sal/system_settings.py`:
- Add comment explanation (pulled from the wiki page) above `EXCLUDED_FACTS` definition
- Add `IGNORE_FACTS` setting with default value of `[]`
- Add `ADD_TO_ALL_BUSINESS_UNITS` with default value of `False`
- Add `ADD_NEW_MACHINES` with default value of `True`
sheagcraig/sal,salopensource/sal,sheagcraig/sal,sheagcraig/sal,salopensource/sal,salopensource/sal,sheagcraig/sal,salopensource/sal