Support for configuring the rabbitmq_management web UI

When rabbitmq::admin_enable => true, then also configure RabbitMQ to enable
the rabbitmq_management web UI, running on rabbitmq::management_port.
Or, if ssl => true, then configure it as an SSL listener on
rabbitmq::ssl_management_port.

Also added a ssl_port parameter which defines the SSL port for RabbitMQ
service itself.  Previously, the rabbitmq::ssl_management_port parameter
was used for the RabbitMQ service SSL bind port.  So this potentially
breaks people who are currently using this module to configure RabbitMQ
for SSL on a non-standard port.  (They need to use the ssl_port parmeter
now, instead of ssl_management_port.)  Not exactly sure the best way
to approach a solution to that.
Okay, you can actually just remove 2.7 from .travis.yml :)