Set variables in rsyslog.conf to disable rate-limiting.

Per Bug https://bugs.launchpad.net/openstack-ci/+bug/1024487, this will
modify the rate-limiting setting for rsyslog.conf.  If rate-limiting is
being used the addition will set it to 0 otherwise the addition will
add the variables to the configuration file and set them to 0.

Implements: Variables "$SystemLogRateLimitBurst 0", and
"$SystemLogRateLimitInterval 0" in "/etc/rsyslog.conf". These changes
are persuent to what has been outlined in the rsyslog docs as found
here : http://www.rsyslog.com/tag/SystemLogRateLimitInterval/

Fixes: Bug 1024487
Change-Id: I2637889cbe9e5e87bbfc0f1bb5047abae34d953c

Updated commit implements changes in code and placement post code
review.
Merge branch 'BUG-1024487' of https://github.com/cloudnull/devstack into BUG-1024487