Add support for parallel stages

[cmmorrow]

Is your feature request related to a problem? Please describe.
Stages with long execution times could benefit from running in parallel as long as they don't depend on one another.

Describe the solution you'd like
Allow for a parallel stage alternative to stage in a config file. By default, parallel stages that are defined sequentially are executed in parallel. Alternatively, a parallel stage can define a group key in which case parallel stages with the same group will be run together.

Additional context
Maybe the vagrant runner shouldn't implement parallel stages since the behavior might not be stable.

Also, a new display type should be considered for use with parallel stages.