Add more flexibility for controlling concurrent pipeline executions

Acceptance Criteria

• User can specify the number of concurrent pipelines that can execute per pipelinerun in the Git repository
• User can specify if all queued pipelinerun should get executed in order or if only the last one should execute and cancel the one that is in-progress
• User should be provided with “Cancel", "GracefullyCancel", and "GracefullyStop" as part of stopping the current pipelinerun