Added memory reservation for ecs

[liorchen]

[mhglover]

One of the test failures is:

cat container_transform/tests/marathon-group.json | container-transform --input-type marathon --output-type compose | tail -18 | head -9
prometheus:
  cpu_shares: 512.0
  image: micahhausler/prometheus-mesos
  mem_limit: 134217728b
  mem_reservation: '[{''uri'': ''https://gist.githubusercontent.com/you/hash/raw/hash/config.yml'',
    ''extract'': False, ''executable'': False, ''cache'': False}]b'
  network_mode: bridge
  ports:
  - 9090:9090

[sveneh]

what's the status of this PR? Will it be merged/fixed? Looking forward to have memory reservation support for ECS. Any possibility to help here?