Move developer setup in a dedicated method

The general idea is to move all the code paths that are NOT for production execution out of configure.

The configure method should start with assert not self.is_read_only. A new method named configure_for_developers would do the opposite: not depending on Arkindex API calls to populate configuration, instead relying on local paths provided by CLI.

The CLI arguments declared on configure can be moved to __init__. The self.add_arguments can be moved to __init__ too.

The choice between BaseWorker.configure and BaseWorker.configure_for_developers will be done in ElementsWorker.configure, depending on self.is_read_only.