In cleanup_mail sysprep check if mail dirs exist

Check if /var/spool/mail exists before trying to remove the content
of the directory.

image_creator/os_type/unix.py

@@ -85,7 +85,9 @@ class Unix(OSBase):
         if print_header:
             self.out.output('Removing files under /var/mail & /var/spool/mail')
 
-        self.foreach_file('/var/spool/mail', self.g.rm_rf, maxdepth=1)
+        if self.g.is_dir('/var/spool/mail'):
+            self.foreach_file('/var/spool/mail', self.g.rm_rf, maxdepth=1)
+
         self.foreach_file('/var/mail', self.g.rm_rf, maxdepth=1)
 
     @sysprep()