Allow a profile to be specified by default

the next logical step would be to define a default_profile ... a profile that is used when no param is used and not profile. In the profile_list command, it could be marked with a * - pretty much like git branches. If you like this idea, could you please create an issue for it.

_Originally posted by @bgruening in https://github.com/galaxyproject/planemo/pull/1102#discussion_r535643131_

Start by locating the profile selection entry point and the profile_list command mentioned in the issue, then trace how profiles are chosen when no parameter is supplied. Done means a configured default_profile is used for that case and profile_list marks the default with an asterisk, with behavior covered by the relevant tests if they exist.

Written by the indexing model from the issue text.