Submit private jobs via API

• depends on !182 (merged)
• updated request_tests in lib/debci/test_handler.rb to update value of is_private field of job as per test object sent by POST /test/:suite/:arch and POST /test/batch.
• updated POST '/test/:suite/:arch/:package' to fetch is_private params for the new job to be created with the check for accepting valid bool values only.
• updated validate_tests and validate_batch_tests in test_handler.rb so that is_private accepts valid bool value only.
• updated after_save callback in job.rb to not update PackageStatus for a private job.
• Added tests in spec/debci/api_spec.rb