Proper handling of image digests

[smoehrle]

Images can be specified like:

nginx
nginx:1.27.0
nginx:1.27.0@sha256:67682bda769fae1ccf5183192b8daf37b64cae99c6c3302650f6f8bf5f0f95df
nginx@sha256:67682bda769fae1ccf5183192b8daf37b64cae99c6c3302650f6f8bf5f0f95df

At the moment ecs-deploy can only handle the second case. With this change, all 4 cases work properly.

[fabfuel]

Hi @smoehrle

Thanks for the nice addition! Would you please add a test like this one which sets a tag with all of your newly supported image digests:

ecs-deploy/tests/test_ecs.py

Lines 530 to 533 in e4f6028

	def test_task_set_tag(task_definition):
	task_definition.set_images(u'foobar')
	for container in task_definition.containers:
	assert container[u'image'].endswith(u':foobar')

Thanks!
Fabian