ipa: check if provided config file exists

Add a parser check to verify config file supplied to the ipa
command exists. Previously, invalid file paths would not results
in any error and would just silently proceed with default config.

https://fedorahosted.org/freeipa/ticket/6114

Reviewed-By: Petr Spacek <pspacek@redhat.com>
Reviewed-By: Martin Basti <mbasti@redhat.com>

ipalib/plugable.py

@@ -44,6 +44,7 @@ from ipalib.text import _
 from ipalib.util import classproperty
 from ipalib.base import ReadOnly, lock, islocked
 from ipalib.constants import DEFAULT_CONFIG
+from ipapython import ipautil
 from ipapython.ipa_log_manager import (
     log_mgr,
     LOGGING_FORMAT_FILE,
@@ -494,6 +495,13 @@ 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):
+                parser.error(
+                    _("%(filename)s: file not found") % dict(filename=value))
+
+            parser.values.conf = value
+
         if parser is None:
             parser = optparse.OptionParser(
                 add_help_option=False,
@@ -517,8 +525,9 @@ class API(ReadOnly):
         parser.add_option('-e', dest='env', metavar='KEY=VAL', action='append',
             help='Set environment variable KEY to VAL',
         )
-        parser.add_option('-c', dest='conf', metavar='FILE',
-            help='Load configuration from FILE',
+        parser.add_option('-c', dest='conf', metavar='FILE', action='callback',
+            callback=config_file_callback, type='string',
+            help='Load configuration from FILE.',
         )
         parser.add_option('-d', '--debug', action='store_true',
             help='Produce full debuging output',