Support load-test scenario on main branch

[HerbertJordan]

The following scenario should be supported by Norma's main branch:

name: Simple Load Test
duration: 120

# The number of static validators present from begin to end
num_static_validators: 4    # default: 1

# Run three different applications consecutively producing different load
applications: 
  - name: loadA
    type: counter
    users: 100
    start: 20
    end: 50
    rate:
      constant: 20

  - name: loadB
    type: counter
    users: 100
    start: 50
    end: 80
    rate:
      constant: 1

  - name: loadC
    type: counter
    users: 100
    start: 80
    end: 110
    rate:
      constant: 100