Unify configuration of jobs and groups

[andrus]

Configuration of jobs and groups should look the same. Instead groups store their children in "jobs" map, and "jobs" - in "dependsOn" list. Let's create a unified recursive configuration like this:

g1:
  type: group
  dependsOn:
    j1: {}
    g2: {}
    j3:
      dependOn: 
         j6: {}
         j7:
            params: 
                 a: b
                 c: d
g2: 
  type: group
  dependsOn: 
    j4: {}
    j5: {}

The new 3.0 runtime should require very little tweaking (mostly tests) to make this work.

This will require more thought. E.g. using job names as keys is too limiting as we may want to have multiple instances of the same job with different parameters. We really need a concept of "execution" in the config