Provide list of valid values for --server option

[pbabinca]

$ dock-pulp -s xxx list
ERROR     Configuration error: pulps section is missing xxx
ERROR     
ERROR     Something is wrong in /etc/dockpulp.conf, or you
ERROR     mistyped an option you are passing to --server

Note - this list should honor the config file so probably it cannot be part of dock-pulp --help.

[yahzaa]

Valid values can be set dynamically by parsing /etc/dockpulp.conf before raising the exception.