Execute a bundle from a pre-computed Run

[carolynvs]

Currently we pass a lot of information to the Porter bundle execution code, and then leave it up to our CNAB runtime implementation to piece it together and execute the bundle. It does make a number of decisions about how to execute the bundle at that late stage.

In order to support workflows, which are composed of pre-computed Run resources, we need to update our CNAB runtime implementation to accept a pre-computed Run and have it execute the instructions encoded on the Run as defined.