stress: Update based on new API
stress: Update based on new API

Stress was erroring out if there were no bridges to delete.
I've fixed that, and I've also improved versbose output so
it prints a `.` for every bridge added, and only counts
additions after the initial population

Signed-off-by: Dave Tucker <bfcdf3e6ca6cef45543bfbb57509c92aec9a39fb@dtucker.co.uk>

socketplane/libovsdb