[Network] Convert VNet Create to use SDK

[Travis Prescott (tjprescott)]

Fixes #2073 (allows multiple address-prefixes for create). Also partially addresses #2198 in that it converts the "FatClient" to use the SDK. There's no need to dynamically generate a template for this command.

The breaking change is the old output format was:
"newVNet": { VNET object }
and now we simply return the VNET object with no need to index on "newVNet".

The resulting object is manually wrapped in "newVNet" so as not to cause a breaking change. This will be removed at the next opportunity of a networking API change.

[Codecov (codecov-io)]

Codecov Report

Merging #2269 into master will decrease coverage by -0.03%.
The diff coverage is 90.32%.

@@            Coverage Diff             @@
##           master    #2269      +/-   ##
==========================================
- Coverage   72.95%   72.92%   -0.03%     
==========================================
  Files         458      441      -17     
  Lines       19499    19308     -191     
  Branches     2578     2730     +152     
==========================================
- Hits        14226    14081     -145     
+ Misses       4390     4305      -85     
- Partials      883      922      +39

Impacted Files	Coverage Δ
...ure/cli/command_modules/network/_client_factory.py	100% <ø> (ø)	✅
...twork/azure/cli/command_modules/network/_params.py	92.51% <100%> (-0.02%)	❌
...work/azure/cli/command_modules/network/commands.py	98.97% <100%> (ø)	✅
...k/azure/cli/command_modules/network/_validators.py	63.32% <100%> (ø)	✅
...twork/azure/cli/command_modules/network/_format.py	35.84% <60%> (+2.51%)	✅
...etwork/azure/cli/command_modules/network/custom.py	59.03% <90.9%> (+0.21%)	✅
...-storage/azure/cli/command_modules/storage/blob.py	14.28% <0%> (ø)	✅
...re-cli-vm/azure/cli/command_modules/vm/commands.py	92.12% <0%> (ø)	✅
...-storage/azure/cli/command_modules/storage/util.py	19.23% <0%> (ø)	✅
src/azure-cli-core/azure/cli/core/prompting.py	36.17% <0%> (ø)	✅
... and 34 more

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[Yugang Wang (yugangw-msft)]

I think required=True is unnecessary