fix(cni): pass buffer to kuma net iptables engine to get logs out

[slonka]

closes #4659

For some reason restoreIPTables returns an empty string when called inside a namespace. This PR uses RuntimeStdout to write to the buffer and then we print the contents of that buffer and that contains iptables rules.

Example log output:

2022-08-15T09:35:53.029Z        DEBUG   kuma-cni        generated iptables rules        {"pod": "test-server-84cdb5864-g4s5r", "namespace": "kuma-test", "podInfraContainerId": "06b35c8a192828555ce2585b6e2dd51ebacece4c70e2a21287c389636901024d", "ip": "", "containerId": "06b35c8a192828555ce2585b6e2dd51ebacece4c70e2a21287c389636901024d", "args": "K8S_POD_NAME=test-server-84cdb5864-g4s5r;K8S_POD_INFRA_CONTAINER_ID=06b35c8a192828555ce2585b6e2dd51ebacece4c70e2a21287c389636901024d;IgnoreUnknown=1;K8S_POD_NAMESPACE=kuma-test", "iptablesStdout": "kumactl is about to apply the iptables rules that will enable transparent proxying on the machine. The SSH connection may drop. If that happens, just reconnect again.
Writing following contents to rules file:  /tmp/iptables-rules-1660556153027488649.txt2074813965
* raw
-A PREROUTING -p udp --sport 53 -j CT --zone 1
-A OUTPUT -p udp --dport 53 -m owner --uid-owner 5678 -j CT --zone 1
-A OUTPUT -p udp --sport 15053 -m owner --uid-owner 5678 -j CT --zone 2
-A OUTPUT -p udp --dport 53 -j CT --zone 2
COMMIT
* nat
-N MESH_INBOUND
-N MESH_OUTBOUND
-N MESH_INBOUND_REDIRECT
-N MESH_OUTBOUND_REDIRECT
-A PREROUTING -p tcp -j MESH_INBOUND
-A OUTPUT -p udp --dport 53 -m owner --uid-owner 5678 -j RETURN
-A OUTPUT -p udp --dport 53 -j REDIRECT --to-ports 15053
-A OUTPUT -p tcp -j MESH_OUTBOUND
-A MESH_INBOUND -p tcp --dport 9901 -j RETURN
-A MESH_INBOUND -p tcp -j MESH_INBOUND_REDIRECT
-A MESH_OUTBOUND -p tcp --dport 9901 -j RETURN
-A MESH_OUTBOUND -s 127.0.0.6/32 -o lo -j RETURN
-A MESH_OUTBOUND -p tcp ! --dport 53 -o lo ! -d 127.0.0.1/32 -m owner --uid-owner 5678 -j MESH_INBOUND_REDIRECT
-A MESH_OUTBOUND -p tcp ! --dport 53 -o lo -m owner ! --uid-owner 5678 -j RETURN
-A MESH_OUTBOUND -m owner --uid-owner 5678 -j RETURN
-A MESH_OUTBOUND -p tcp --dport 53 -j REDIRECT --to-ports 15053
-A MESH_OUTBOUND -d 127.0.0.1/32 -j RETURN
-A MESH_OUTBOUND -j MESH_OUTBOUND_REDIRECT
-A MESH_INBOUND_REDIRECT -p tcp -j REDIRECT --to-ports 15006
-A MESH_OUTBOUND_REDIRECT -p tcp -j REDIRECT --to-ports 15001
COMMIT

iptables set to diverge the traffic to Envoy.
"}

Signed-off-by: slonka slonka@users.noreply.github.com

Checklist prior to review

• Link to docs PR or issue -- no docs on new CNI yet
• Link to UI issue or PR -- not ui issue
• Is the issue worked on linked? --
• The PR does not hardcode values that might break projects that depend on kuma (e.g. "kumahq" as a image registry) --
• The PR will work for both Linux and Windows, system specific functions like syscall.Mkfifo have equivalent implementation on the other OS --
• Unit Tests -- do I need to write this or E2E tests? I don't see anywhere tests that would stub / mock log so not sure how much effort is needed there - let me know if you want me to do this
• E2E Tests --
• Manual Universal Tests -- not a universal issue
• Manual Kubernetes Tests -- tested manually (see example log above)
• Do you need to update UPGRADE.md? -- no
• Does it need to be backported according to the backporting policy? -- no

[codecov-commenter]

Codecov Report

Merging #4826 (b0188fb) into master (767315c) will decrease coverage by 0.02%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #4826      +/-   ##
==========================================
- Coverage   46.47%   46.45%   -0.03%     
==========================================
  Files         690      690              
  Lines       47059    47061       +2     
==========================================
- Hits        21873    21863      -10     
- Misses      23264    23274      +10     
- Partials     1922     1924       +2     

Impacted Files	Coverage Δ
app/cni/pkg/cni/injector_linux.go	0.00% <0.00%> (ø)
pkg/plugins/leader/postgres/leader_elector.go	82.97% <0.00%> (-6.39%)	⬇️
pkg/core/resources/manager/cache.go	83.11% <0.00%> (-5.20%)	⬇️
pkg/xds/cache/once/cache.go	82.05% <0.00%> (-5.13%)	⬇️
pkg/xds/cache/once/once.go	95.00% <0.00%> (-5.00%)	⬇️

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[bartsmykla]

lgtm