Do not use the env proxy by default and check_rabbitmq_cluster script

Hi James,

I added theses following changes to the plugin:

#### Do not use the env proxy by default
By using the proxy env by default, you can’t tell to any check scripts to don’t use proxy at all. The only way would be to empty the proxy url by `--proxyurl= ""`

Now by calling the aliveness script:
`./check_rabbitmq_aliveness -H localhost` => we don’t use the env proxy
`./check_rabbitmq_aliveness -H localhost --proxy` => we use the env proxy

I think it's more logical this way.

#### Add check_rabbitmq_cluster script
I added the `check_rabbitmq_cluster` script. It checks how many node are in the cluster and throw an warning or a critical alert if the node number are bellow the threshold.

Partition are critical, but sometimes we have many nodes and wanted an critical alerte only if we have an critical number of nodes. So I've just written quickly this script to do that.

What do you think about those changes?