[rabbitmq_consumers] Fix rabbitmqctl output filter

[codeurimpulsif]

Goal

• Fix rabbitmqctl output filter to avoid bad values

Informations

By default rabbitmqctl list_queues -p / name consumers give this kind of output:

Timeout: 60.0 seconds ...
Listing queues for vhost / ...
name    consumers
celery  1

Then in this plugin it result to:

Timeout:.value 60.0
name.value consumers
celery.value 1

The consumers value generates the following error message:

[ERROR] In RRD: Error updating /var/lib/munin/production/server1-rabbitmq_consumers-name-g.rrd: /var/lib/munin/production/server1-rabbitmq_consumers-name-g.rrd: Function update_pdp_prep, case DST_GAUGE - Cannot convert 'consumers' to float

This merge request fix the grep command to get this plugin result:

celery.value 1

[codeurimpulsif]

Is this PR ok? Did I need to add other things?