Add external clusters support

[maciaszczykm]

What this PR does / why we need it:

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #2604

Special notes for your reviewer:

Release note:

Added external clusters support.

[kubermatic-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: maciaszczykm

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• OWNERS [maciaszczykm]

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[kubermatic-bot]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
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[maciaszczykm]

/assign @floreks

[codecov]

Codecov Report

Merging #2608 into master will decrease coverage by 0.49%.
The diff coverage is 25.35%.

@@            Coverage Diff             @@
##           master    #2608      +/-   ##
==========================================
- Coverage   52.91%   52.42%   -0.50%     
==========================================
  Files         232      234       +2     
  Lines        8079     8208     +129     
  Branches      989     1004      +15     
==========================================
+ Hits         4275     4303      +28     
- Misses       3801     3902     +101     
  Partials        3        3              

Impacted Files	Coverage Δ
...nt-details/machine-deployment-details.component.ts	40.17% <0.00%> (ø)
...app/core/services/page-title/page-title.service.ts	21.53% <0.00%> (-3.03%)	⬇️
src/app/shared/entity/cluster.ts	63.55% <0.00%> (-0.55%)	⬇️
src/environments/environment.ts	100.00% <ø> (ø)
src/app/shared/utils/node-utils/node-utils.ts	4.16% <4.16%> (ø)
src/app/core/services/cluster/cluster.service.ts	17.85% <17.77%> (-2.56%)	⬇️
...r/cluster-details/node-list/node-list.component.ts	79.20% <25.00%> (+12.82%)	⬆️
...app/cluster/cluster-list/cluster-list.component.ts	67.22% <40.00%> (-8.27%)	⬇️
...mponents/external-cluster-data-dialog/component.ts	47.36% <47.36%> (ø)
...luster/cluster-details/version-picker/component.ts	81.53% <100.00%> (ø)
... and 5 more

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[maciaszczykm]

/retest

[maciaszczykm]

/retest

[floreks]

/lgtm

[kubermatic-bot]

LGTM label has been added.

Git tree hash: d5acc62ec194ddb84dc9c2c605599520cb6614eb

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[maciaszczykm]

/retest

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/retest
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[kubermatic-bot]

@kubermatic-triage-bot:

In response to this:

/retest
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Silence the bot with an /lgtm cancel or /hold comment for consistent failures.

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