Make TempestConf a subclass of SafeConfigParser

TempestConf was needlessly re-implementing or overwriting the behavior of
SafeConfigParser and now the duplication was removed. The check of whether the
value is a string or unicode in `TempestConf.set` has been removed, since
SafeConfigParser already does that. The `TempestConf.is_modified` method was
removed, since it was deceiving (i.e., not actually checking if the key was
modified, only if it is already configured).
Refactor service availability setting
Switch service names and code names
Refactor API version setting

This could have been done trough tempest/config.py, but that causes it to load
various tempest functionality, like exception printing on the log output.
Add --verbose option and set use the info level

The logging.DEBUG level should be kept for truly low level information and
logging.INFO for things the user might actually want to know.
Improved info and debug messages