Attempt to detect chef-server-core along with private-chef

Would appreciate feedback on more Ruby-Idiomatic ways to do this, but:

1.  `pkg_provider.load_current_resource` raises an `Chef::Exceptions::Package` if `chef-server-core` is installed and you query for `private-chef`  -- I think this is because of [1] below.   So rescue on that and return `0.0.0` which is the equivalent of uninstalled

2.  Modify the `private_chef_installed_version` method to fall back to `chef-server-core` if `private-chef` isn't installed.



[1]
```
root@backend1:~# apt-cache showpkg chef-server-core
Package: chef-server-core
Versions: 
12.0.0-rc.6-1 (/var/lib/dpkg/status)
 Description Language: 
                 File: /var/lib/dpkg/status
                  MD5: a5a909d9290df04b6b8e337089019c28


Reverse Depends: 
Dependencies: 
12.0.0-rc.6-1 - private-chef (0 (null)) private-chef:i386 (0 (null)) private-chef (0 (null)) private-chef:i386 (0 (null)) 
Provides: 
12.0.0-rc.6-1 - 
Reverse Provides: 
```
Looks good.  This is a totally ruby-esc way to do it.

It might be slightly more readable as:
return private_chef_version unless private_chef_version == '0.0.0'
self.installed_version('chef-server-core', node)

But this is totally readable and gets a :+1: from me as it is.  Nice patch.  Thank you.