apply, rollback: Revert to a state where IPv6 is disabled

In cases where the previous state on an interface included IPv6
disabled and the processing of the desired state caused it to be
enabled, the rollback failed to re-disable the IPv6 stack.

In case of a rollback, the previous state is processed and IPv6 is
disabled if needed.

Signed-off-by: Edward Haas <edwardh@redhat.com>

libnmstate/netapplier.py

@@ -22,6 +22,7 @@ from contextlib import contextmanager
 import copy
 import logging
 import six
+import time
 
 from libnmstate import metadata
 from libnmstate import netinfo
@@ -31,6 +32,7 @@ from libnmstate import state
 from libnmstate import sysctl
 from libnmstate import validator
 from libnmstate.error import NmstateConflictError
+from libnmstate.error import NmstateError
 from libnmstate.error import NmstateLibnmError
 from libnmstate.error import NmstatePermissionError
 from libnmstate.error import NmstateValueError
@@ -150,6 +152,14 @@ def _apply_ifaces_state(
         raise NmstatePermissionError('Error creating a check point')
     except nm.checkpoint.NMCheckPointCreationError:
         raise NmstateConflictError('Error creating a check point')
+    except NmstateError:
+        # Assume rollback occured, revert IPv6 stack state.
+        # Checkpoint rollback is async, there is a need to wait for it to
+        # finish before proceeding with other actions.
+        # TODO: https://nmstate.atlassian.net/browse/NMSTATE-103
+        time.sleep(5)
+        _disable_ipv6(current_state)
+        raise
 
 
 def _create_editable_desired_state(