Attempting to support other Task variables in order to support FARGATE launch type

[mlibrodo]

In order to support the fargate launch type, the cpu, memory, execution_role, task_execution_role_arn needs to be specified in RegisterTaskDefinitionInput. I just went ahead and exposed these to drone. i believe it should address issue #27

Sample drone config

  deploy:
    image: mlibrodo/drone-ecs:latest
    when:
      branch: master
    access_key: XXXXXXXX
    secret_key: XXXXXXXX
    region: us-east-2
    family: task-definition-family
    docker_image: namespace/repo
    container_name: container1
    tag: latest
    cluster: some-fargate-cluster
    service: some-service
    task_network_mode: awsvpc
    port_mappings:
      - 9000 9000
    deployment_configuration: 50 200
    log_driver: awslogs
    log_options:
      - awslogs-group=/ecs/someservice
      - awslogs-region=us-east-2
      - awslogs-stream-prefix=ecs
    environment_variables:
      - SERVICE_CONFIG=XXXXX
    cpu: 4096
    memoryReservation: 128
    task_cpu: 4096
    task_memory: 8192
    task_execution_role_arn: arn:aws:iam::XXXXX:role/ecsTaskExecutionRole
    compatibilities: FARGATE EC2
    desired_count: 2

[josmo]

Seems reasonable to me. Thanks for the contribution @mlibrodo