Always handle config files with Cuttlefish

[dumbbell]

This has several benefits:

1. It simplifies the code, all configuration being handled by the same code path (no more condition on Erlang-term-based vs. Cuttlefish). rabbit_config shrinks quite a lot in the process.

2. We can use additional configuration files AND an Erlang-term-based configuration file. In other words, it is possible to use the same existing Erlang-term-based file and introduce Cuttlefish files when needed.

   It allows a user to run RabbitMQ with:

   RABBITMQ_CONFIG_FILE=/path/to/rabbitmq.config \
   RABBITMQ_CONFIG_FILES=/path/to/conf.d/*.conf \
   ./sbin/rabbitmq-server
   

   A developer can do the same with make run-broker:

   make run-broker \
   RABBITMQ_CONFIG_FILES=/path/to/conf.d/*.conf
   

   In the example above, the main configuration file generated by rabbitmq-run.mk is an Erlang-term-based one.

This is implemented by calling Cuttlefish with a (possibly empty) list of additional files and the Erlang-term-based file as the advanced configuration file.

References #2180.

[gerhard]

Works as expected, confirmed by running the following command locally:

cat > rabbitmq.conf <<EOF
load_definitions = /Users/gerhard/github.com/rabbitmq/3.9.x/deps/rabbit/rabbitmq-definitions.json
EOF

cat > rabbitmq-definitions.json <<EOF
{
  "global_parameters": [
    {"name": "cluster_name", "value": "pr-2277"}
  ]
}
EOF

RABBITMQ_CONFIG_FILES=/Users/gerhard/github.com/rabbitmq/3.9.x/deps/rabbit/rabbitmq.conf make run-broker
...
(rabbit@focker)1> rabbit_mnesia:status().
[{nodes,[{disc,[rabbit@focker]}]},
 {running_nodes,[rabbit@focker]},
 {cluster_name,<<"pr-2277">>},
 {partitions,[]}]