Add get a single pipeline tests

What does this MR do?

Adds 'Get a single pipeline' API and Web tests and a new setting in Project Config file - pipeline_sha.

By default there are 11 pipelines in gitlabhq project. Both tests require to know pipeline ID for get requests. However when importing a project to GitLab environment, a different pipeline ID is being assigned depending on the total amount of pipelines the instance has. Thus the tests are using commit SHA of the pipeline to pull the pipeline's ID.

Related issues

Closes #412 (closed)

Checklist

• Ensure test data for the new endpoint is representative. We recommend using large data ("edge cases") in performance tests. The impact of optimizations is more visible on the big numbers and large data may expose more performance bottlenecks.
• Fill in @endpoint, @description, and @gpt_data_version tags with the information regarding the new test.
  • @gpt_data_version should be generally 1 unless you know the test is for data that's in a specific version.
• Create a bug if the endpoint response time is above the performance main target (500 ms).
  • Specify created issue link under @issue tag.
• Consider specifying the additional @flags:
  • unsafe - required for Unsafe tests. Ensure that data before and after the test is consistent.
  • search - labels Search related tests.
  • dash_url - labels Web tests with the check if the endpoint path has a dash \ redirect.
• Web tests
  • Specify all called controller methods under the @description.
  • Consider creating MR to add the new test to GitLab Performance SiteSpeed -> performance-sitespeed#10.