Gitlab CB v6.1.21

Maintenance/bugfix release

  * Add node['database']['userhost'] attribute, fixes #57
  * Fix gitlab['listen_port']/gitlab['https'] condition, fixes #58
  * Add node['gitlab']['self_signed_cert'] and documentation, fixes #58
  * Revert yum-epel changes, COOK-4164 blocking, will re-open
    - Fix dependency hell w/ nginx cookbook
  * Add rubocop linting
  * Add chefspec mysql/postgres http/https branching specs
  * Remove unneeded python dependency