add a .gitignore file
add a test-requirements.txt file
use a ChainMap to store the configuration
add the mechanism to read and merge config files

I used the same mechanism as xivo-ctid for the config files
the main config file /etc/xivo-agent/config.yml is maintained by the packager and should
not be modified by the administrator.

Extra configuration files stored in /etc/xivo-agentd/conf.d override content from the
main configuration file and will be in alphabetical order or priority
use read_config_file_hierarchy from lib-python
remove unused imports
add debian packaging for the config file
remove foreground from default config file

reason: foreground has no real utility outside of CLI interface
add comments in default config file
fix extra config dir name