Http api

Use http api of rabbitmq server to collect queue statistics data to avoid issue said here:
https://github.com/phrawzty/rabbitmq-collectd-plugin/issues/5
In this issue, rabbitmqctl failed to be spawn inside collectd python plugin even though the python script can be run without problem in system python environment. Although the root cause has not been identified yet, http api seems to be a clean way to collect the data.