Support multiple images update

[gfronza]

First of all, thanks for this nice project! It already helped me a lot!

These changes add the possibility of updating multiple containers of the given task definition in a single deployment. For that, I had to replace the --image argument for container_image (multiple values are allowed).

Without this, task definitions with multiple containers could not be updated all at once. Plus, it was always replacing the image of the first container (containerDefinitions[0]).

Thanks again.

[coveralls]

Coverage increased (+1.5%) to 51.079% when pulling f00a841 on gfronza:master into 3f282cd on cuttlesoft:master.

[coveralls]

Coverage increased (+0.05%) to 49.65% when pulling e5311cd on gfronza:master into 3f282cd on cuttlesoft:master.

[frankV]

Hey @gfronza thanks for this. @katie7r and I will review and merge this in ASAP.

[coveralls]

Coverage increased (+0.7%) to 50.345% when pulling 2192227 on gfronza:master into 3f282cd on cuttlesoft:master.

[coveralls]

Coverage increased (+0.7%) to 50.345% when pulling b064dae on gfronza:master into 3f282cd on cuttlesoft:master.