don't use EOF as default value for insertafter

This patch fixes issue #4700.

```
- lineinfile: dest=/etc/libvirt/libvirtd.conf line="auth_unix_ro = \"none\"" regexp="\#auth_unix_ro = \"none\""
```

Using this task with every run a new line ```auth_unix_ro = "none"``` is attached at the EOF of /etc/libvirt/libvirtd.conf. That's not the expected behavior. The expected behavior is that the line ```#auth_unix_ro = "none"``` is replaced by ```auth_unix_ro = "none"```.

Looks like the issue is that ```insertafter = 'EOF'``` is used when not setting insertafter and insertbefore.