Fixes bug 923894
Previously all stdout/err from glance services was discarded
if launched via glance-control.
The --capture-output option allows such output to be instead
captured in the syslog.
Non-termination output should be flushed to ensure timely
arrival in /var/log/messages.
Change-Id: I5c11ad4b9f7f321a3d1cab71e34c2a6707ef1b5e