Seeing this in ironic-conductor.log:
018-07-18 16:14:02.638 7 ERROR oslo_service.service [req-cdc2bb58-3a04-4f7d-a5a6-2dbbc55341f6 - - - - -] Error starting thread.: DriverLoadError: Driver, hardware type or interface ilo-pxe could not be loaded. Reason: [Errno 13] Permission denied: '/var/lib/ironic/httpboot/boot.ipxe'.

2018-07-18 16:14:02.638 7 ERROR oslo_service.service Traceback (most recent call last):
2018-07-18 16:14:02.638 7 ERROR oslo_service.service   File "/usr/lib/python2.7/site-packages/oslo_service/service.py", line 792, in run_service
2018-07-18 16:14:02.638 7 ERROR oslo_service.service     service.start()
2018-07-18 16:14:02.638 7 ERROR oslo_service.service   File "/usr/lib/python2.7/site-packages/ironic/common/rpc_service.py", line 54, in start
2018-07-18 16:14:02.638 7 ERROR oslo_service.service     self.manager.init_host(admin_context)
2018-07-18 16:14:02.638 7 ERROR oslo_service.service   File "/usr/lib/python2.7/site-packages/ironic/conductor/base_manager.py", line 140, in init_host
2018-07-18 16:14:02.638 7 ERROR oslo_service.service     self._collect_periodic_tasks(admin_context)
2018-07-18 16:14:02.638 7 ERROR oslo_service.service   File "/usr/lib/python2.7/site-packages/ironic/conductor/base_manager.py", line 268, in _collect_periodic_tasks
2018-07-18 16:14:02.638 7 ERROR oslo_service.service     for ifaces in driver_factory.all_interfaces().values():
2018-07-18 16:14:02.638 7 ERROR oslo_service.service   File "/usr/lib/python2.7/site-packages/ironic/common/driver_factory.py", line 271, in all_interfaces
2018-07-18 16:14:02.638 7 ERROR oslo_service.service     return {iface: interfaces(iface) for iface in _INTERFACE_LOADERS}
2018-07-18 16:14:02.638 7 ERROR oslo_service.service   File "/usr/lib/python2.7/site-packages/ironic/common/driver_factory.py", line 271, in <dictcomp>
2018-07-18 16:14:02.638 7 ERROR oslo_service.service     return {iface: interfaces(iface) for iface in _INTERFACE_LOADERS}
2018-07-18 16:14:02.638 7 ERROR oslo_service.service   File "/usr/lib/python2.7/site-packages/ironic/common/driver_factory.py", line 262, in interfaces
2018-07-18 16:14:02.638 7 ERROR oslo_service.service     return _get_all_drivers(_INTERFACE_LOADERS[interface_type]())
2018-07-18 16:14:02.638 7 ERROR oslo_service.service   File "/usr/lib/python2.7/site-packages/ironic/common/driver_factory.py", line 323, in __init__
2018-07-18 16:14:02.638 7 ERROR oslo_service.service     self.__class__._init_extension_manager()
2018-07-18 16:14:02.638 7 ERROR oslo_service.service   File "/usr/lib/python2.7/site-packages/oslo_concurrency/lockutils.py", line 274, in inner
2018-07-18 16:14:02.638 7 ERROR oslo_service.service     return f(*args, **kwargs)
2018-07-18 16:14:02.638 7 ERROR oslo_service.service   File "/usr/lib/python2.7/site-packages/ironic/common/driver_factory.py", line 393, in _init_extension_manager
2018-07-18 16:14:02.638 7 ERROR oslo_service.service     on_missing_entrypoints_callback=missing_callback))
2018-07-18 16:14:02.638 7 ERROR oslo_service.service   File "/usr/lib/python2.7/site-packages/stevedore/named.py", line 81, in __init__
2018-07-18 16:14:02.638 7 ERROR oslo_service.service     verify_requirements)
2018-07-18 16:14:02.638 7 ERROR oslo_service.service   File "/usr/lib/python2.7/site-packages/stevedore/extension.py", line 203, in _load_plugins
2018-07-18 16:14:02.638 7 ERROR oslo_service.service     self._on_load_failure_callback(self, ep, err)
2018-07-18 16:14:02.638 7 ERROR oslo_service.service   File "/usr/lib/python2.7/site-packages/ironic/common/driver_factory.py", line 378, in _catch_driver_not_found
2018-07-18 16:14:02.638 7 ERROR oslo_service.service     raise exception.DriverLoadError(driver=ep.name, reason=exc)
2018-07-18 16:14:02.638 7 ERROR oslo_service.service DriverLoadError: Driver, hardware type or interface ilo-pxe could not be loaded. Reason: [Errno 13] Permission denied: '/var/lib/ironic/httpboot/boot.ipxe'.
2018-07-18 16:14:02.638 7 ERROR oslo_service.service 
2018-07-18 16:14:03.538 7 INFO ironic.common.rpc_service [-] Stopped RPC server for service ironic.conductor_manager on host undercloud.localdomain.