EC2: Implement CreateDefaultSubnet operation

[viren-nadkarni]

This PR implements the EC2 CreateDefaultSubnet operation.

CreateDefaultSubnet operation creates a default subnet with a size /20 IPv4 CIDR block in the specified Availability Zone in your default VPC. You can have only one default subnet per Availability Zone.

Moto already supports the EC2 CreateDefaultVpc and this is updated such that it also creates the default subnets.

The implementation assigns the /20 CIDR blocks to AZs in sequence as follows, but that's not always the case on AWS:

• ...a: 172.31.0.0/20
• ...b: 172.31.16.0/20
• ...c: 172.31.32.0/20
• etc.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.82%. Comparing base (b29a9c8) to head (f01d97b).
Report is 33 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8818      +/-   ##
==========================================
+ Coverage   92.80%   92.82%   +0.01%     
==========================================
  Files        1261     1266       +5     
  Lines      109888   110530     +642     
==========================================
+ Hits       101986   102599     +613     
- Misses       7902     7931      +29     

Flag	Coverage Δ
servertests	28.16% <71.79%> (-0.06%)	⬇️
unittests	92.79% <100.00%> (+0.01%)	⬆️

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[bblommers]

LGTM - thanks for improving the parity here @viren-nadkarni!