Need to suppress expected command failures

[Walt Boring]

Currently privsep will always log all stderr from commands executed here:

https://github.com/openstack/oslo.privsep/blob/master/oslo_privsep/daemon.py#L284

When attaching a volume to a Nova instance, os-brick tries to run commands to collect connector information. When a system doesn't have a Fibre Channel HBA on the system it's expected that running "systool -c fc_host -v" will result in a failure. There is no reason to log this as a warning. You can see the output here:

http://openstack-3rd-party-storage-ci-logs.virtuozzo.com/43/190843/26/check/dsvm-tempest-kvm/3d395b4/logs/screen-n-cpu.txt.gz

It should be up to the caller to decide if they want to log the stderr output, not privsep.