support Iptables 1.8 with legacy and nf_tables support

[murali-reddy]

Fixes #3465

• upgrade to alpine image (for amd64 only) with iptables 1.8.3
• updated alpine image bundles both (legacy and nftable) version iptables, iptables-save, iptables-restore
• by default iptables* commands soft linked to iptables-legacy*
• user can explicitly specify the iptable backed
• have borrowed back-end detection logic from kube-proxy

need to test with Kubernetes 1.17 which has kube-proxy in nft mode

[bboreham]

Should we do anything for the non-Kubernetes case? I.e. the weave script.

[murali-reddy]

need to test with Kubernetes 1.17 which has kube-proxy in nft mode

tested the auto detection logic with Kubernetes 1.17(kube-proxy and kubelet using iptables nft mode) and Debain buster (iptables with nftable as default)

Should we do anything for the non-Kubernetes case? I.e. the weave script.

We could ask users to use iptables-legacy on the host (for e.g.) for the moment so even update to the base image should work fine. Alternatively we could soft-link to appropriate binaries in the weave container during weave launch

[bboreham]

Since Alpine appears to use symbolic links (ln -s), I think we should follow suit, just in case it causes confusion.

[bboreham]

please add this var to docs

[murali-reddy]

Updated base branch to 2.6 and addressed review comments.