Update client, requests, and services to support specifying worker image variant used for job

[robertbartel]

Update the client, CLI parsing, message classes, and service logic to enable users to specify a particular image tag to be used for a requested job. To facilitate this, also add CLI, client, service, etc. functionality for listing what options are currently available deferring addition of listing functionality to #715; see comment below.

Note that #658, #661, and #662 are all closely related.

[robertbartel]

Deferring on the additional functionality to list available options until a later, separate issue (#715).

One, it isn’t essential. Users can always get by with the implied default. They also have other practical ways of figuring out what they need (e.g., docker image ls). If they are interested in specifying a worker version, it's quite likely they will know how to use something else that will give them the right version/tag name.

Two, doing it properly is neither small in scope nor clear in approach. There are many practical configuration issues that would either need to be hard-coded or overhauled in our config constructs (something that probably needs doing for other reasons). And that is assuming a direct query from the client to the registry, which may not be appropriate. It may be better to go through the request service for such a query, which would further complicates the design. Even beyond all that, there are implications that must be considered (much of which are currently unknown) regarding the plans to add support for other infrastructure backends beyond Docker Swarm.

The right thing to do at this point is rely on current error condition behavior when the expected image for a job is not actually available in the registry. That functionality isn’t especially robust at the moment either, but technically that in and of itself is already a pre-existing bug (there is not a guarantee that, e.g., 127.0.0.1:5000/ngen:latest would be available, even if that is the only, hard-coded image that can be used). As such, that also deserves its own issue: #716.