add --robot-paths configure option

@geimer: this will allow you to add the easyconfigs archive/repo path to the list of robot paths, without always having the robot enabled.

We can consider including the easyconfigs repo path by default for EasyBuild v2.0, people can easily kick it out by customising `--robot-paths` in their EB configuration (although we would need a way to indicate the path to the installed easyconfigs package then without hardcoding it, hmm...).

This needs further testing and some enhanced unit tests, but it should be working.
this should be if/else: robot_path can be an empty list (if default is empty string and eb is invked with `--robot`). should this throw an error, since there are no paths to use the robot on?