Filter workloads by container name in fluxctl

[ogerbron]

This PR adds an optional argument to fluxctl list-workloads. It gives the ability to filter workloads with a specific container. This is especially useful when using micro services with different processes, rather than parsing fluxctl output with shell scripting.

For example:

fluxctl list-workloads --container my-container
WORKLOAD                                                   CONTAINER  IMAGE                                   RELEASE  POLICY
default:deployment/micro-service-process-1   my-container  docker/my-container:master  ready    automated
default:deployment/micro-service-process-2   my-container  docker/my-container:master  ready    automated
default:deployment/micro-service-process-3   my-container  docker/my-container:master  ready    automated

I am quite new to Go programming, so do not hesitate to correct me or give me hints on how I can be a better contributor :).

[2opremio]

Can you please update the fluxctl documentation?