Allow a custom file mode when setting up debugging

This will be handy in the future if we will want to install or uninstall
only single IPA components and want to append to the installation logs.
This will be used by the upcoming ipa-dns-install script.

ipaserver/install/installutils.py

@@ -145,14 +145,14 @@ def port_available(port):
 
     return rv
 
-def standard_logging_setup(log_filename, debug=False):
+def standard_logging_setup(log_filename, debug=False, filemode='w'):
     old_umask = os.umask(077)
     # Always log everything (i.e., DEBUG) to the log
     # file.
     logging.basicConfig(level=logging.DEBUG,
                         format='%(asctime)s %(levelname)s %(message)s',
                         filename=log_filename,
-                        filemode='w')
+                        filemode=filemode)
     os.umask(old_umask)
 
     console = logging.StreamHandler()