[ssl] allow to manage content of certs/keys files

Allow to use two ways of certs/keys files management:

1) via specifing a path of a file:

   ssl:
      key_file: /etc/rabbitmq/ssl/key.pem

2) via specifing a path and content of a file:

   ssl:
       key: text
       key_file: /etc/rabbitmq/ssl/key.pem

The files management via authority must be removed as an unused.

Change-Id: Icdc8783449cf8ac1283f107861564b6ad36230ec
8 files changed