Use os.path.isfile() and isdir()

Replace custom file_exists() and dir_exists() functions with proper
functions from Python's stdlib.

The change also gets rid of pylint's invalid bad-python3-import error,
https://github.com/PyCQA/pylint/issues/1565

Signed-off-by: Christian Heimes <cheimes@redhat.com>
Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>

ipalib/plugable.py

@@ -515,7 +515,7 @@ class API(ReadOnly):
         Add global options to an optparse.OptionParser instance.
         """
         def config_file_callback(option, opt, value, parser):
-            if not ipautil.file_exists(value):
+            if not os.path.isfile(value):
                 parser.error(
                     _("%(filename)s: file not found") % dict(filename=value))