Allow setting of submission metadata.

Currently the deployment descriptions as output in the command `nomad deployment list` are defined as constants in the Nomad [structs](https://github.com/hashicorp/nomad/blob/master/nomad/structs/structs.go#L4091) package. It would be nice if an operator can optionally set a custom description for deployments; allowing the Nomad tracking of deployments to become more personalised. This could be particularly helpful on clusters with numerous customers and for dashboarding.

Start with the deployment description definitions in nomad/structs/structs.go and trace how the `nomad deployment list` command obtains and displays them. Determine where an operator-provided description should enter the deployment metadata flow. Done means deployments can optionally carry a custom description and `nomad deployment list` displays it for tracking and dashboarding.

Written by the indexing model from the issue text.