Major cli overhaul, add skip nginx_config during initialisation

[tobiasraabe]

MERGE AFTER #104

What is included:

• overhauled cli (this needs to be tested)
• split settings in static and environment related
• no ovmm_conf.yml anymore, only bash script ovmm_conf

Todo:

• Separate parsers and validators
• Revisit HISTORY.rst
• Add more documentation

PR

• Reference Create BaseCommand class #4, "add_user" + sudo #100, Upgrade of user static commands #107, Comments on the oTree Virtual Machine Image and OVMM #111 (partial), Add skip for nginx on initialise #113, File exists error #114, Error message despite 'sudo' #115, click 2.0 update causes legacy error #116

[tobiasraabe]

This seems stable now. We have to test it.