Use endpoint to load Azure subnets

[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 #2919

Special notes for your reviewer:

Release note:

Use endpoint to load Azure subnets autocompletions.

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[codecov]

Codecov Report

Merging #2988 (5a5130d) into master (622b3a1) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #2988      +/-   ##
==========================================
- Coverage   51.87%   51.85%   -0.02%     
==========================================
  Files         237      237              
  Lines        8326     8329       +3     
  Branches     1022     1022              
==========================================
  Hits         4319     4319              
- Misses       4004     4007       +3     
  Partials        3        3              

Impacted Files	Coverage Δ
src/app/core/services/wizard/provider/azure.ts	12.61% <0.00%> (ø)
src/app/core/services/api/service.ts	5.11% <0.00%> (-0.09%)	⬇️

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[maciaszczykm]

/assign @kgroschoff @floreks

This adds the subnets which are last resource for Azure. Additionally, I've fixed the struct for VNets as by mistake I've used wrong key.

[kgroschoff]

/lgtm

[kubermatic-bot]

LGTM label has been added.

Git tree hash: 5f824590185e8da1743b8d5955b7a62b99124d2c