[20.10] libnetwork: skip firewalld management for rootless

[AkihiroSuda]

Fix moby/moby#43781

"Cherry-pick" https://github.com/moby/moby/pull/43793

- What I did Fix:

• No networking in rootless docker with firewalld #43781

- How I did it

Skip firewalld management when running with RootlessKit

- How to verify it

sudo systemctl enable firewalld
dockerd-rootless-setuptool.sh install -f
docker --context=rootless run --rm alpine sh -euxc 'apk add neofetch && neofetch'

- Description for the changelog

libnetwork: skip firewalld management for rootless

- A picture of a cute animal (not mandatory but encouraged) 🐧

[AkihiroSuda]

cc @thaJeztah