Add custom key content & service provider support

* Custom service provider types allow RabbitMQ being
  installed by Puppet in a much more flexible ways,
  an HA configurations with Pacemaker, for example.
  (Note, that for given example with Pacemaker, Pupet
  service type should be additionally enhanced by
  new provider type for Pacemaker)

* Custom key source/content for Debian (apt module) is
  required in order to allow rabbitmq being installed
  by Puppet for intranet only faced hosts as well as
  internet faced ones.
  It also elliminates a hardcode for URL which is a kind of
  SPOF and 'limitation' for operators' freedom of choose.

* Update docs as well

DocImpact
Implements improvement: #MODULES-1407
Implements improvement: #MODULES-1408

Signed-off-by: Bogdan Dobrelya <bdobrelia@mirantis.com>