A task in a workflow can now have volume mounts

[gauravgahlot]

The intention here is to specify all the volumes at the task level, that are common for all
actions within that task. The volumes at the task level will be mounted in all actions. However,
if a user has the same mount at the action level, it will get a higher preference and will
override the one at the task level.

Example:

version: '0.1'
name: worklfow-sample
global_timeout: 600
tasks:
- name: "task-one"
  worker: "{{index .Targets "machine1" "mac_addr"}}"
  volumes:
    - /dev:/dev
    - /dev/console:/dev/console
    - /lib/firmware:/lib/firmware
  actions:
  - name: "action-one"
    image: action-one
    timeout: 90
  - name: "action-two"
    image: action-two
    timeout: 300
    volumes:
      - /lib/firmware:/lib/firmware:ro