Add generate-config script and document config options

[Azrenbeth]

Builds on #402. Fixes #101.

Stop using CONFIG_DEFAULTS dictionary

• Replace in code with config.get(option, default_value)
• List of available options (and defaults) can now be found in new documentation file (docs/configuration.md)

Stop generating config file on first run of Sydent. For reasons brought up in #401. This will not affect people with existing config files, only new installers.

• Add a generate-config script that creates a minimal config.
• Add an example config file (docs/example_config.conf)

---

Not sure if I did the correct thing:

• currently, the environment variables that Sydent reads (e.g. SYDENT_PID_FILE) are used to define some default config options ONLY on the first time sydent is run (or if you edit the config file to delete the things that Sydent adds automaticall)
• Now, those defaults are available to be set using the generate-config script (e.g. the --pid-file argument) with the environment variables taking precedence. But if the user's config file does not have a value set, then the environment variable is used.
• As an example:
  1. I run generate-config with no arguments
  2. I run sydent which uses sydent.pid to store the process ID
  3. I set SYDENT_PID_FILE to somefile.pid
  4. I run sydent which uses somefile.pid to store the process ID

An alternative would have been to require those values to be set by the config file.

[babolivier]

Note that since @Azrenbeth has finished his internship with us it's likely someone else will have to take this over.

[clokep]

I'm removing review on this since it depends on #402, which is a sizeable PR.