YAML file as database back-end for kotnetcli-server

[jovanbulck]

Different CGI scripts on kotnetcli test server should communicate via a simple YAML file, acting as a persistent database back-end. To ensure deterministic test behavior, we want to start up with "clean"/known database state every time we re-start server.py. I therefore propose to:

1. create an init_db.yml file in kotnetcli/server with known dummy state
2. before starting the CGIHTTPServer, server.py copies this file into a new temporary file, say tmp_db.yml (see https://docs.python.org/2/library/tempfile.html)
3. on cgi-bin/wayf2.pl GET request for a specific user, the returned pwdxxx and a timestamp are saved into tmp_db.yml (as a yaml attribute for that user)
4. on cgi-bin/netlogin.pl POST request, the info from tmp_db.yml is used to validate the request; on success, the IP address and other relevant info is stored in tmp_db.yml (as a yaml attribute for that user)