Add key_content parameter for offline install

* 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-1408

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

Fix package_gpg_key key_source and docs

* Reuse package_gpg_key for apt as well
* Update/add docs for package_gpg_key, key_source
* Fix key_source usage in repo::apt

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