More detailed instructions for etcd-setup

[simonferquel]

Etcd-operator default behavior makes etcd clusters creation really fast, at the cost of not being secure, nor Highly available.

This PR adds details about this in the etcd deployment doc. It also adds pointers to etcd-operator documentation to enable persistent storage, node anti-affinity and mutual TLS.

This should fix #71

[simonferquel]

(requested @ulyssessouza and @ijc for reviewers, to check if the doc is ok, without compose on k8s experience)

[codecov-io]

Codecov Report

Merging #73 into master will decrease coverage by 1.09%. The diff coverage is n/a.

@@            Coverage Diff            @@
##           master      #73     +/-   ##
=========================================
- Coverage   65.78%   64.69%   -1.1%     
=========================================
  Files          88       88             
  Lines       10941    10941             
=========================================
- Hits         7198     7078    -120     
- Misses       3455     3591    +136     
+ Partials      288      272     -16

Impacted Files	Coverage Δ
internal/controller/resourceupdater.go	1.85% <0%> (-42.6%)	:arrow_down:
api/compose/v1alpha3/stack.go	0% <0%> (-40%)	:arrow_down:
internal/stackresources/stackstate.go	44.73% <0%> (-23.69%)	:arrow_down:
...i/client/clientset/typed/compose/v1alpha3/stack.go	20.83% <0%> (-17.71%)	:arrow_down:
internal/controller/stackreconciler.go	51.06% <0%> (-15.96%)	:arrow_down:
internal/controller/controller.go	87.5% <0%> (-12.5%)	:arrow_down:
internal/convert/stack.go	72.89% <0%> (-8.42%)	:arrow_down:
internal/controller/stackownercache.go	87.3% <0%> (-4.77%)	:arrow_down:
internal/convert/service.go	97.24% <0%> (-1.84%)	:arrow_down:
internal/convert/pod.go	86.12% <0%> (-1.44%)	:arrow_down:

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[simonferquel]

Updated with your comments. @silvin-lubecki @ijc did I miss something ?

[simonferquel]

@chris-crone can you review/merge ?