Show image IDs as given by the user

[squaremo]

If I use a DockerHub-hosted image, say postgres, flux shows this in output as

tluafed/users-db                users-db             index.docker.io/library/postgres              
                                                     |   9.2-alpine                                10 Aug 17 23:11 UTC
                                                     |   9.2.22-alpine                             10 Aug 17 23:11 UTC
                                                     |   9.2                                       10 Aug 17 23:08 UTC
                                                     |   9.2.22                                    10 Aug 17 23:08 UTC
                                                     |   9.3-alpine                                10 Aug 17 23:07 UTC
                                                     |   9.3.18-alpine                             10 Aug 17 23:07 UTC
                                                     |   9.3                                       10 Aug 17 23:04 UTC
                                                     |   9.3.18                                    10 Aug 17 23:04 UTC
                                                     |   9.4-alpine                                10 Aug 17 23:03 UTC
                                                     |   9.4.13-alpine                             10 Aug 17 23:03 UTC
                                                     '-> 9.4                                       10 Aug 17 23:01 UTC

This is meeeostly harmless, but I can see it leading to confusion: copying and pasting that image with another version tag into fluxctl release ... gives

$ fluxctl $LOCAL release --service=tluafed/users-db --update-image=index.docker.io/library/postgres:9.2
Submitting release ...
Nothing to do

[samb1729]

Implemented in #817.