Simplify access to maintenance scripts

[pbordron]

Scripts dgenies/bin/clean_jobs.py and dgenies/bin/local_scheduler.py are not easily accessible. For example, the full path of clean_jobs.py in a virtualenv is the following:

/usr/local/venv/dgenies/lib/python3.6/site-packages/dgenies-1.5.0-py3.6.egg/dgenies/bin/clean_jobs.py

Possibles solutions:

• complete dgenies program with something like dgenies clean-jobs (duplicate with dgenies clear ?) and dgenies local-scheduler, or
• edit setup.py in order to add those scripts in script section (by prefixing script names with a dgenies- prefix)

See #63 #68

[pbordron]

Maybe rename local_scheduler.py into dgenies-scheduler.py

Also maybe only add maintenance scripts in PATH if package installed with dgenies[web] or dgenies[drmaa]
In maintenance mode, it is may be not required as there is no db