Added support for nginx domain aliases

This commit add new feature: gitlab_domain_alias, This feature
make it possible to also configure dns aliases for the nginx
server name.

I use this, because i need the server to be acessible by gitlab_domain
(git.example.com) and  git hostname without the domain.

The default behaviour is gitlab_domain_alias disabled (false), so this
would not break anyone that expect the old behaviour (only gitlab_domain)
in nginx server_name.

There is example on how to use this feature on init.pp and test coverage.

If you want to use gitlab_domain_alias just configure the parameter
Only one alias
This will create the following  nginx server_name:
class {'gitlab':
       gitlab_domain       => "git.example.com",
       gitlab_domain_alias => "git"
      }
Multiple aliases
This will create the following nginx server_name:
class {'gitlab':
       gitlab_domain       => "git.example.com",
       gitlab_domain_alias => "git gitlab gitlab.example.com"
      }

(Cherry-pick from commit 1f7fe246a00329262a6b270c54a883f222a48304)
Update changelog