allow propertysources to set any property

I ran into this recently trying to deal with the groups-are-always-for-security issue.  My goal was to use a simple tag string to identify the location, then have those strings map to the actual location string (so they don't all need to be manually updated if moved).  So far, I have managed location properties for devices by always placing each into a Location group, but since this breaks security even though that group has nothing to do with security, we have to stop using that method.  What I found was the propertysource documentation is ambiguous (and even has an example of setting "location" via SNMP), but support confirmed ALL properties are prepended with "auto."  They came up with a workaround using the API, but it would be nice if there was some way to just create arbitrary properties via the properysource mechanism.  Perhaps there should be some indicator that "I really mean it", like using a "." prefix or something similar.  At the very least, the documentation should be updated to clarify the limitations.