Add initial test module for save command
Split sources into plugins and groups into subdir

Each sources is a separate plugin now.
Each group is configured by separate file now.
Add basic testing for `save` via command line.
Add debug level `-l` and make tests independent

The debug level option `-l` works for both `ndc` and `test` command.
Tests use unique directories every time to be independent of
residual files left on the system.
Add test for groups configuration files
Break config into separate files for all groups
Get log level from env var if not cli-specified
Move `windows_event_logs` source into plugin
Move `rpm_list` source into plugin
Move `env_list` source into plugin
Move `sys_confs` source into plugin
Move `sys_logs` source into plugin
Move `sys_storage` source into plugin
Move `sysstat_logs` source into plugin
Move `fresh_sysstat_logs` source into plugin
Fix sources configuration for consistency
Allow plugins have plug-able test modules
Merge branch 'save'

- Finish fixing basic `save` command functionality.
- Convert all sources/plugins (executed by `save`) into
 separate plugins.
- Configure groups in separate files under separate directory.