Allow summoners to be configurable

Allow setting the name of the plugin that contains the RoboHydra
picker function to use, so that if there's more than one, one of
them can be specified in the configuration to allow the server to
start.
Allow plugins without heads or tests

Instead, check that plugins don't contain unknown properties. That
was (probably) the real reason behind checking that there were at
least some head or test, it's more effective, and it allows for
empty plugins, which sometimes are useful (ie. the logger plugin).