Multi-container support

[arschles]

Currently, apps are only allowed to have 1 container in them. Multi-container apps should be supported. It's trivial to enable them in a pod, but the interface via the CLI will need to be figured out. Also need to decide on a few more things:

• Should we allow each container to have its own env. vars?
• Should each container get to expose its own ports?
• If so, how should we route those ports internally?
  • And if we do, should we allow other apps to talk to this one?

Additionally, we might want to add specific sidecars "built in" to enable things like logging adapters. some ideas:

• Logging sidecar to send logs to splunk, papertrail, azure monitor, stackdriver, etc...
• Opentracing sidecar
• A service mesh sidecar