updated config.yaml; add .gitignore; add backup directory for simplestreams; small fix in python code

Merge branch 'master' of https://git.garr.it/cloud/operations/update-images

* update requirements
* add keystone and regions parameters to generate-files-for-simplestream.py

2019-02-20:  FG;  Update image list, add Fedora-29 and Ubuntu-18.04, remove CentOS-6 Debian-8 Ubuntu-14.04.

2018-09-28:  FG;  Add hw_rng_model decoration to recent images: should be match by hw_rng:allowed=True on Nova flavors.

2018-09-14:  FG;  1) handle image parameters like min_size,min_ram 2) handle tags within extraargs 3) do not remove old image, rather rename - set private - update description.