obmc-phosphor-systemd.bbclass: Update configparser module class name

There is a warning message in the obmc-phosphor-systemd.bbclass file
when using bitbake to build image-bmc:
```
DeprecationWarning: The SafeConfigParser class has been renamed to
ConfigParser in Python 3.2. This alias will be removed in future
versions. Use ConfigParser directly instead.
  parser = configparser.SafeConfigParser(strict=False)
```
Use the class name `ConfigParser` directly in the configparser module.

Signed-off-by: George Liu <liuxiwei@inspur.com>
Change-Id: I6168d9d1100ac846cc31be837a20b7dedb6d7910