flannel, ovn and multus network plugins did not support all taint keys. This
update changes the tolerations to support them all.

According to the documentation:

```
There are two special cases: An empty key with operator Exists matches all keys,
values and effects which means this will tolerate everything. An empty effect matches
all effects with key key.
```

Usage of the empty `key` and `effect` ensures the network plugin daemonset will
be deployed on every nodes (ex: in case of custom taints, or NoExecute effect)