Add use_syslog to Marconi config

This patch adds use_syslog option to the marconi config file. This
is needed to allow marconi to run, when USE_SCREEN is set to False
in devstack.

Change-Id: I547697ec2745975e235a4e58cde81132ac37b70d
Merge "Add use_syslog to Marconi config"
Merge branch 'master' of github.com:openstack-dev/devstack