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

Fix for project without group

See merge request !192