privsep: lack of capabilities on kernel 4.15

cinder-backup failed at least on Ubuntu 16.04 with kernel v.4.15.
I tested with Queens and newer but looking into the code, it could affect older versions too.

sep.daemon [-] privsep log:
vsep.daemon [-] privsep log: Original exception was:
vsep.daemon [-] privsep log: Traceback (most recent call last):
vsep.daemon [-] privsep log: File "/usr/bin/privsep-helper", line 10, in <module>
vsep.daemon [-] privsep log: sys.exit(helper_main())
vsep.daemon [-] privsep log: File "/usr/lib/python2.7/dist-packages/oslo_privsep/daemon.py", line 5
vsep.daemon [-] privsep log: LOG.exception(e)
vsep.daemon [-] privsep log: File "/usr/lib/python2.7/logging/__init__.py", line 1473, in exception
vsep.daemon [-] privsep log: self.logger.error(msg, *args, **kwargs)
vsep.daemon [-] privsep log: File "/usr/lib/python2.7/logging/__init__.py", line 1193, in error
vsep.daemon [-] privsep log: self._log(ERROR, msg, args, **kwargs)
vsep.daemon [-] privsep log: File "/usr/lib/python2.7/logging/__init__.py", line 1286, in _log
vsep.daemon [-] privsep log: self.handle(record)
vsep.daemon [-] privsep log: File "/usr/lib/python2.7/logging/__init__.py", line 1296, in handle
vsep.daemon [-] privsep log: self.callHandlers(record)
vsep.daemon [-] privsep log: File "/usr/lib/python2.7/logging/__init__.py", line 1336, in callHandl
vsep.daemon [-] privsep log: hdlr.handle(record)
vsep.daemon [-] privsep log: File "/usr/lib/python2.7/logging/__init__.py", line 759, in handle
vsep.daemon [-] privsep log: self.emit(record)
vsep.daemon [-] privsep log: File "/usr/lib/python2.7/logging/handlers.py", line 414, in emit
vsep.daemon [-] privsep log: sres = os.stat(self.baseFilename)
vsep.daemon [-] privsep log: OSError: [Errno 13] Permission denied: '/var/log/cinder/cinder.log'