Added possibility to switch to iptables nf_tables

[cheina97]

Description

This PR adds the possibility of forcing the usage of iptables legacy/nf_tables using the env variable IPTABLES_MODE which accept 2 values:

• legacy
• nf_tables

If not set the used iptables mode will be the system default (legacy in case of alpine)

This PR uses a fork of the official go-iptables package while waiting for the PR
coreos/go-iptables#110 to be merged.

How Has This Been Tested?

• Locally on KinD

[adamjensenbot]

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[cheina97]

/rebase test=true

[cheina97]

/test

[cheina97]

/rebase test=true

[cheina97]

/merge