Run Scope-aaS probes directly in ECS

[2opremio]

Currently, the simplest way to run Scope (and Scope-aaS) in ECS is through the Weave ECS AMI or cloud formation templates.

This is a pretty hard requirement for existing ECS clusters since running a different AMI requires respawning all the EC2 instances in the cluster (which in turns may require changing a Launch Configuration ....)

For Weave Net/Run (and even for Scope non-aaS) the AMI still makes sense since Weave needs to influence how the ecs-agent is initialized (in order for it to talk to the Weave Proxy) and instances require extra permissions to obtain the Weave/Scope instance peers (see https://github.com/weaveworks/guides/blob/d1039713f6b65c21c59dac309e1b3ae962c478bb/aws-ecs/data/weave-ecs-policy.json#L16-L17 )

For Scope-aaS we only need to to run a probe in each instance (without any awareness of the other instances) configured with a specific Scope-aaS probe token so we could perfectly run it directly on ECS.

It would also be good to make this mechanism work with the existing ECS AMI so that users can upgrade to Scope-aaS without reinstalling.

[2opremio]

One of the reasons for using ECS AMIs and Cloud formation templates is that ECS didn't provide an easy way to schedule one Scope/Weave agent per machine.

Since yesterday it is possible to do so through daemon scheduling.