Add cbagent to the perfrunner manifest

Change-Id: Ie37e5e9b42812cdf62f6b113a58053e6e4bf6eac
Reviewed-on: http://review.couchbase.org/43210
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>