rabbitmq::manage_repos sets 'ensure' in apt::source

The ensure paramenter in apt::source is set to 'present' or 'absent'
based on the value of rabbitmq::manage_repo.
Added rspec tests for previous commit 4dad6be
Rename variable 'manage_repos' to 'repos_ensure'

'manage_repos' doesn't describe the intended behaviour of the variable.
We decided to rename it to 'repos_ensure'. This will break backward
compatibility.
Introduced backward compatibility for $manage_repos

$manage_repos was initially proposed for removal. This was making
this module backward incompatible. This commit reintroduces
$manage_repos, introducing a deprecation warning if used and
falling back to the $ensure_repos behaviour.
Added rspec tests/$manage_repo backward compatible

Added some rspec tests after reintroducing $manage_repo in order to make
the module backward compatible, while still retaining the use of
$repos_ensure.
Merge pull request #209 from queeno/ensure_apt_source

manage_repos sets 'ensure' in apt::source