Use selected engine version when running backtest in the cloud

[jhonabreul]

Expected behaviour

Running a backtest in the cloud with lean cloud backtest "Project" should use the selected engine version for the project.

Actual behaviour

The backtest is run using master, regardless of whether the user selects a different version in QC Cloud

Potential solution

The API should use the user selected engine to run backtests instead of master. Additionally, the API could allow indicating the engine version to use.

[jhonabreul]

Closing as it's a duplicate of #152 (see #152 (comment))