DNS: Reinitialize DNS resolver after changing resolv.conf

Previously the installer did not reinitialize resolver so queries for
records created using --ip-address option might not be answered. This led
to incorrect results during 'Updating DNS system records' phase at the
end of installation.

This is kind of hack but right now we do not have enough time to extend
python-dns's interface with resolver_reinit() method.

https://fedorahosted.org/freeipa/ticket/5962

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

ipaserver/install/bindinstance.py

@@ -17,6 +17,7 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #
 
+from __future__ import absolute_import
 from __future__ import print_function
 
 import tempfile
@@ -27,6 +28,7 @@ import re
 import sys
 import time
 
+import dns.resolver
 import ldap
 import six
 
@@ -982,6 +984,10 @@ class BindInstance(service.Service):
             resolv_fd.close()
         except IOError as e:
             root_logger.error('Could not write to resolv.conf: %s', e)
+        else:
+            # python DNS might have global resolver cached in this variable
+            # we have to re-initialize it because resolv.conf has changed
+            dns.resolver.default_resolver = None
 
     def __generate_rndc_key(self):
         installutils.check_entropy()