Add ability to run plans (not just steps) sequentially

[rpappalax]

ported over from loads-broker: loads/loads-broker#112

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 for these to verify one feature.

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.