support use of %(DEFAULT_ROBOT_PATHS)s template in EasyBuild configuration files

This is required to provide users full control over which robot paths are used, while retaining the possibility of using the installed easyconfig files as a fallback (or not).
`options.robot_paths` already is a list, rather than a `:`-separated string, and it's possible that there are additional paths listed (if other paths containing `easybuild/easyconfigs` are found in  Python search path), so checking for equality is not reliable