Fix Generator retries for project and group create calls

Follow-up on !234 (merged)

We should flush response body before retry sending a call to create group/project when using persistent connection.

Otherwise retry will fail with exception:

[2020-08-21 17:46:31] INFO: Error creating project 'gpt-project-719' (Attempt 1) - status 500 Internal Server Error
[2020-08-21 17:46:32] ERROR:
GPT data generation failed:
Tried to send a request while one is pending already. Make sure you read off the body.
 Traceback:["/usr/local/bundle/gems/http-4.4.1/lib/http/connection.rb:71:in `send_request'", "/usr/local/bundle/gems/http-4.4.1/lib/http/client.rb:74:in `perform'", "/usr/local/bundle/gems/http-4.4.1/lib/http/client.rb:31:in `request'", "/usr/local/bundle/gems/http-4.4.1/lib/http/chainable.rb:27:in `post'", "/performance/lib/gpt_test_data.rb:287:in `block (4 levels) in create_projects'", "/performance/lib/gpt_test_data.rb:286:in `times'", "/performance/lib/gpt_test_data.rb:286:in `block (3 levels) in create_projects'", "/performance/lib/gpt_test_data.rb:264:in `times'", "/performance/lib/gpt_test_data.rb:264:in `block (2 levels) in create_projects'", "/performance/lib/gpt_test_data.rb:261:in `each'", "/performance/lib/gpt_test_data.rb:261:in `each_with_index'", "/performance/lib/gpt_test_data.rb:261:in `block in create_projects'", "/usr/local/bundle/gems/http-4.4.1/lib/http/chainable.rb:147:in `persistent'", "/performance/lib/gpt_test_data.rb:260:in `create_projects'", "/performance/lib/gpt_test_data.rb:326:in `create_horizontal_test_data'", "./bin/generate-gpt-data:115:in `<main>'"]