Feature multiple ldap push authentication

Add proper multi server authentication

In addition, to reduce complexity, I've moved the authentication to it's own class.

See merge request !194

When trying to instantiate a new, but unknown provider, guidance is provided to get the correct provider

RuntimeError: Unknown provider (henk). Available providers: ["ldapmain"]

Conflicts:
	app/models/user.rb
	config/gitlab.yml.example
	spec/lib/gitlab/ldap/access_spec.rb

Note: authentication agains an other server, other then the first
still does not work. This will be completed in further MR's

It now just returns the results of the first server. A new call will replace
the existing to get results for a specific provider

- Use smarter instance methods
- Support multiple LDAP servers

More new code from CE 7.4 into EE

See merge request !191