Use -m flag on rabbitmq-plugins command for managing rabbitmq_plugin resources

Use the -m flag for calls to the rabbitmq-plugins command, which enables minimal
output mode.  This is much easier and more reliable to parse.

Fixes an issue under RMQ 3.4.0 where rabbitmq_plugin resources were being enabled
on every Puppet run, even though they are already enabled.
Merge pull request #254 from misterdorm/pluginparsing

Use -m flag on rabbitmq-plugins command for managing rabbitmq_plugin res...