Fixed GetOSFromSeries to detect MacOSX

This PR, fixes: https://github.com/juju/juju/issues/887
Hi @axw,

I agree. A better fix for this would be to separate the client code from the agent code. As it stands now, there are many global variables that define agent paths which get compiled in the client as well.

Until there was multi OS support in the agent, all global variables were simply hardcoded, and although the client never used those paths, it still had them compiled in and initialized on startup. 

If I remember corectly, there was an effort at some point to remove globals throughout the codebase and turn them into functions that would get called only when really needed (in the agent code). Not sure how that concluded.

In regards to defaulting to linux paths, there is a high chance that most of the OSs we will support (on the agent side) will be linux based. So this would save us from adding:

```go
case version.Debian:
    return linuxVals[valname], nil
case version.CentOS:
    return linuxVals[valname], nil
case version.Suse:
    return linuxVals[valname], nil
```