Streamline command handlers

 * Review and edit all command line option definitions
 * Remove AboutCommand since Click bug is now fixed
 * Implement cnsistently --quiet contract
 * Update and rename common error and error logging functions
 * Return proper exit code on errors #362
 * Rename attrib --vartext option to --variable and
   add validation callback and parsing function
 * Add validation callback to inventory --filter option

Signed-off-by: Philippe Ombredanne <[email protected]>

Author: pombredanne