EC2: add SecurityGroupArn to security groups

[pinzon]

Motiviation

As stated in the docs the methods for creating and describing a Security Group resource should include the attribute SecurityGroupArn.

Changes

• ARN generation in the SecurityGroup model
• Addition of the attribute in the response templates.

Testing

• Extension of an existing test to assert the presence of the attribute

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.59%. Comparing base (57d0c0e) to head (cc825a7).
Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8393   +/-   ##
=======================================
  Coverage   94.59%   94.59%           
=======================================
  Files        1159     1159           
  Lines      101447   101448    +1     
=======================================
+ Hits        95964    95965    +1     
  Misses       5483     5483           

Flag	Coverage Δ
servertests	28.86% <100.00%> (+<0.01%)	⬆️
unittests	94.56% <100.00%> (+<0.01%)	⬆️

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[bblommers]

LGTM - thank you @pinzon!