Add sys.passwd, sys.shadow and sys.group variables

Add sys.passwd, sys.shadow and sys.group which exapand to the location of the password, shadow and group files across multiple platforms

```
body common control
{
    bundlesequence => { "passwd" };
}

bundle agent passwd
{
    reports:

        freebsd::

            "sys.passwd = $(sys.passwd)";
            "sys.shadow = $(sys.shadow)";
            "sys.group  = $(sys.group)";
}
```

```
skreuzer@clamps:~
$ sudo cf-agent -f ./passwd.cf -K
R: sys.passwd = /etc/passwd
R: sys.shadow = /etc/master.passwd
R: sys.group  = /etc/group
```