Install iptables package based on the k0s version

[twz123]

Only install iptables if the k0s version requires it. Mark the corresponding host method as deprecated, as that code path can probably be completely removed in upcoming versions.

Fixes #363.

[kke]

I've been trying to think of some clean way to use different strategies based on different k0s versions but haven't come up with a good one yet.

Something like:

strategy := versionstrategy.NewStrategy(
  versionstrategy.LessThan("v1.25.0"), func() {
    foofoo
  },
  func()  {
    barbar
  },
)
strategy.Run()

Or maybe just some helper like:

if p.Config.Spec.K0s.VersionLessThan("v1.25.0") {
   foofoo
}

Or maybe better for readability:

const k0sVersionWithFooFeature = version.MustCompile("v1.25.0")

...
...

if k0sVersionWithFooFeature.GreaterOrEqual(p.Config.Spec.K0s.Version) {
  legacyfoofoo
}

or perhaps:

const fooFeatureConstraint = versionconstraint.MustCompile(">= v1.25.0")

...
...

if fooFeatureConstraint.SatisfiedBy(p.Config.Spec.K0s.Version) {
  foofoo
}

[till]

Maybe treat it as a general feature gate/flag thing? You could use it to enable/disable features too.

I think a => v1.2.3 style constraint is great. Most flexible. No need for less than or anything in code? In code you could use registry.IsEnabled("foo") and the basic scaffolding is:

type FeatureRegistry struct {
  // container for all flags
  Version string // p.Config.Spec.K0s.Version
}
func (f *FeatureRegistry) IsEnabled(feature string) bool {
  // fetch feature by its name from the registry
  // version constrain code here
  return true
}
func (f *FeatureRegistry) Add(feature FeatureGate) error {
  // for initial/global setup
  return nil
}
type FeatureGate struct {
  Name string
  Constraint versiontypestringsomething
}

[kke]

I think k0sctl still works for something like k0s v0.10.0 before the scheme switch.

The upgrade smoke currently starts from v1.21.6+k0s.0.

Should we start removing other stuff that was added to make k0sctl work with versions that are now unsupported? cc: @jnummelin