Split registration

[mudler]

This PR completes the work about splitting the registration stage of EiriniX - apparently there were some pending items, so every extension now can split and decide on their own when to register the mutating webhooks. It also includes fixes to issues found during the development.

It also adds the integration test suite, which runs using kind , adding two simple test cases:

1) A simple extension which just injects an env var - it register and starts in one step (as it is now, default behavior) 2) The same extension is registered, and then later on started. It should still mutate the fake Eirini app pods.

Let's see now how travis/codecov likes it :) I think it should be also possible to account the integration tests into the coverage, I'll try to have a look

[codecov[bot]]

Codecov Report

Merging #16 into master will increase coverage by 0.3%. The diff coverage is 53.84%.

@@            Coverage Diff            @@
##           master      #16     +/-   ##
=========================================
+ Coverage   70.78%   71.09%   +0.3%     
=========================================
  Files           3        3             
  Lines         421      422      +1     
=========================================
+ Hits          298      300      +2     
+ Misses         99       98      -1     
  Partials       24       24

Impacted Files	Coverage Δ
webhook.go	76.71% <100%> (-0.32%)	:arrow_down:
manager.go	63.31% <40%> (+0.88%)	:arrow_up:

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