Merge branch 'issue-ldap-initialization' into 'master'

Issue ldap initialization

If LDAP is disabled error will appear:

```
ERROR -- : Missing setting 'servers' in 'ldap' section in /Volumes/Work/gitlab/development/community/gitlab/config/gitlab.yml (Settingslogic::MissingSetting)
```

@marin Can you test if this will solve the startup problem?

See merge request !1179

config/initializers/7_omniauth.rb

-module OmniAuth::Strategies
+if Gitlab::LDAP::Config.enabled?
+  module OmniAuth::Strategies
     server = Gitlab.config.ldap.servers.values.first
     const_set(server['provider_class'], Class.new(LDAP))
-end
+  end
 
-OmniauthCallbacksController.class_eval do
+  OmniauthCallbacksController.class_eval do
     server = Gitlab.config.ldap.servers.values.first
     alias_method server['provider_name'], :ldap
+  end
 end
\ No newline at end of file

config/initializers/devise.rb

@@ -204,7 +204,7 @@ Devise.setup do |config|
   #   manager.default_strategies(scope: :user).unshift :some_external_strategy
   # end
 
-  if Gitlab.config.ldap.enabled
+  if Gitlab::LDAP::Config.enabled?
     Gitlab.config.ldap.servers.values.each do |server|
       if server['allow_username_or_email_login']
         email_stripping_proc = ->(name) {name.gsub(/@.*$/,'')}