Fix misspelled classname in posix

Whenever the get_os_utils is called on a unix system, it will try to load the PosixUtils class,
but the name of the class is misspelled as PoxixUtil, therefore not being able to find it.

Change-Id: Ia9b67527b7af3282e3c4658fbfd22fae28b7e2e7

cloudbaseinit/osutils/posix.py

@@ -17,7 +17,7 @@ import os
 from cloudbaseinit.osutils import base
 
 
-class PosixUtil(base.BaseOSUtils):
+class PosixUtils(base.BaseOSUtils):
 
     def reboot(self):
         os.system('reboot')