Add ability to run plans (not just steps) sequentially

[rpappalax]

Currently we only have the ability to run steps back-to-back, but there are projects for which we have multiple scenarios and would also like to run "plans" back to back.

Example:
screenshots (pageshot) has 4 loadtesting scenarios, each with 4 steps.
We'd like to group the steps for each scenario together as plans but run them all sequentially.