Change default strategy to serial

[simonvane]

What would you like to be added: It is my understanding that the default strategy for phases and steps is parallel. I propose this be changed to serial.

Why is this needed: It was surprising to me that phases and steps in plans were deployed in parallel by default rather than serial.

Parallel is something that is more typically opted into because it is much more likely to cause issues than running in serial. Running in parallel is usually a well-considered optimisation.