fix: add an attribute to examples/completed to avoid name conflict in test concurrent runs

[gberenice]

what

• Adds an attribute to the examples/competed root module - the MD5 hash of a timestamp. Thit adds a "random" attribute to the resources and helps to avoid resource name conflict during the concurrent runs, [e.g.]:

  Error: creating CloudWatch Logs Log Group (/aws/lambda/eg-test-sns-default): operation error CloudWatch Logs: 
  CreateLogGroup, https response error StatusCode: 400, RequestID: 1eefd044-082e-4b8e-8c3b-0b3c3f2dd8a8, 
  ResourceAlreadyExistsException: The specified log group already exists
  

• Bumps terraform-aws-modules/notify-slack/aws to support newer Python version before 3.8 EOL

why

• Tests are failing.
• See the initial discussion in this thread chore(deps): update terraform terraform-aws-modules/notify-slack/aws to v6 - autoclosed #57 (comment)

[gberenice]

/terratest

[gberenice]

/terratest