Add rabbitmq plugins support

[dineshbhor]

This PR adds support of specifying rabbitmq plugins parameters
through galaxy role.

[sibradzic]

@dineshbhor
Hi there! Thanks for the PR. I'd like to suggest some changes here:

• Since this uses actually leverages https://docs.ansible.com/ansible/latest/modules/rabbitmq_parameter_module.html, the doc should indicate that this is not just plugin-specific
• The case where plugin is not deployed at all by rabbitmq_plugins but parameters are set should be tested

And, yes, the travis test should be able to pass in order to merge the change :)