ListPlugins has a lot of code that is only used for printing the
output to stdout, which is only used by PluginLsCommand.
Simplify ListPlugins to only return the map of plugins, and move the
logic for displaying the output into PluginLsCommand.
PluginLsCommand does not have access to the Plugin objects, but only
to what is returned in PluginInfo instances. Extend that to include
the filename, which was previously accessible to ListPlugins.
This change removes support for the format option from the list
plugins REST API endpoint.
Change-Id: I0c352e587d5f0e8d524ae5b7322e1feec9434d58