Install Weave dynamically on Convox

[2opremio]

Right now, in order to get Weave to work in Convox you need to install the Weave ECS AMI: http://blog.weave.works/2015/09/18/using-weave-and-scope-with-convox/

For existing Convox installations this requires re-installing, which is an unpleasant requirement.

So, we would like to be able to install Weave on existing Convox installations:

• For Weave Net and Weave Run you can't easily do this since the ecs-agent needs to talk to the Weave proxy instead of the Docker daemon. Making this happen requires access to the Convox instances which is not easily automatable right now. Getting https://github.com/convox/rack/issues/16 implemented would make it easier but unfortunately it's prioritized as "Icebox" (https://trello.com/c/FcoXrwLC/140-instance-level-commands)
• For Weave Scope, we may be able to run it as a Convox service. This may be particularly simple for Scope-aaS (since it just requires installing the probes)

[2opremio]

#23 and #24 would make this much simpler.