Optimize Browbeat Workflow by allowing Mix-And-Match workloads and tasks

Browbeat allows multiple workload providers but currently executes them in the order they are passed in via the CLI. We would like to "extend" this by restructuring the browbeat workload portion of the browbeat configuration file. Restructuring would solve open issues such as implementing a method to adjust the cloud during a browbeat run. This should also make adding additional workload providers easier in the future.

Example workload/tasks yaml structure:

tasks:
  - name: Authenticate-1process
    enabled: true
    type: rally
    file: rally/authenticate/keystone-cc.yml
    times: 10000
    concurrency:
      - 32
      - 64
      - 128

  - name: Adjust-keystone-worker-count
    enabled: true
    type: ansible
    playbook: ansible/browbeat/adjustment-workers.yml
    extra_vars: "service=keystone workers=24 threads=1"

  - name: Authenticate-24processes
    enabled: true
    type: rally
    file: rally/authenticate/keystone-cc.yml
    times: 10000
    concurrency:
      - 32
      - 64
      - 128

  - name: fio
    enabled: true
    type: perfkit
    benchmarks: fio
    image: centos7
    machine_type: m1.small
    os_type: rhel
    openstack_image_username: centos
    openstack_volume_size: 20
    openstack_floating_ip_pool: browbeat_public
    openstack_network: browbeat_private