[cli] Check if a plugin is loaded before reloading it again

Before loading a plugin check if it is already loaded. This could
happen when a command has a redirect (ie: set_target redirects to
login), as it will call runner again and try to load again the same
plugins. If we load them again we'll get an error message saying
plugin is loaded but it didn't contain any command.

Change-Id: I7896334c54e0e70b18baf0e4339566a4c361a7c4