Delete headers for list-images and list-workloads

[ogerbron]

This PR adds an optional argument to fluxctl list-images and list-workloads. It gives the ability to delete the list header. This is especially useful when using fluxctl in shell scripts.

For example:

$> fluxctl list-workloads --no-headers
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]

Please rebase on master instead of merging master in

[2opremio]

Also, please update the fluxctl documentation and make sure that CI succeeds.

[2opremio]

Suggested change

	For those using `fluxctl` in scripts, have a look at `--no-headers` for both `list-images` and `list-workloads` command to suppress the header:
	When using `fluxctl` in scripts, you can remove the table headers with `--no-headers` for both `list-images` and `list-workloads` command to suppress the header: