Chosing the python interpreter should be based on the package version, not the machine series

On a xenial host with a backported landscape-client from bionic, the hooks fail to execute because:

```
[...]
interpreter="/usr/bin/python"

if [ `lsb_release -rs` \> "18.00" ]; then
    # landscape-client has py3 libs from bionic forward
    interpreter="/usr/bin/python3"
fi
[...]
```

Instead, we should check the package version:
```
[...]
interpreter="/usr/bin/python"

if [ `dpkg -s landscape-client | awk '/^Version: / {print $2}'` \> "18.00" ]; then
    # landscape-client has py3 libs from bionic forward
    interpreter="/usr/bin/python3"
fi
[...]
```