Suppress testing the API endpoint from the command-line for select commands (e.g. import:prepare)

Changes:
* add a additional method and variable to the CommandOption class
* mark commands that don't require connectivity (API, toolbelt or
  JAR update indifferently) in the 'add_list' factories of list.rb
* separate validation and test for the API endpoint
* if the endpoint is provided as -e / --endpoint, test
  connectivity if the command requires it
* if the endpoint is setup with the server:endpoint command,
  test the connectivity immediately
* if the endpoint comes from the configuration file, connectivity
  has been checked before, the test is not repeated