Implement a configuration mechanism to support the new configuration syntax.

[vkarak]

• Configuration file lookup. We want ReFrame to look into different standard locations for a configuration file.
• Update framework's internals and glue with the new configuration infrastructure.

[vkarak]

These are the steps for the configuration front end.

1. Find configuration file
   a. Check if specified in command line
   b. Check if $HOME/.reframe/settings.{py,json,yaml}
   c. Check in $REFRAME_INSTALL_PREFIX/settings.{py,json,yaml}
   d. Check in /etc/reframe.d/settings.{py,json,yaml}
2. Pick config file parser
   a. Python
   b. JSON
   c. YAML
   d. Old Python config (deprecated)
3. Parse file and load configuration
4. Validate configuration
5. Detect system and reduce configuration: this step produces a json configuration for the current system only after it has resolved the system references in the different configuration sections.
6. Modify configuration based on command line and environment variables
7. Update with defaults
8. Initialize logging
9. Initialize printer
10. Initialize runtime