Parse SSO authentication config on startup

Signed-off-by: Andrej Shadura <andrew.shadura@collabora.co.uk>

src/api/config/auth.yml

+fdo-gitlab:
+  strategy: gitlab
+  description: Freedesktop.org GitLab
+  scope: read_user openid profile email
+  client_id: hexhexhexhex
+  client_secret: hexhexhexhex
+  client_options:
+    site: https://gitlab.freedesktop.org/api/v4

src/api/config/initializers/omniauth.rb

+OmniAuth.config.path_prefix = '/user/sso'
+
+path = Rails.root.join("config", "auth.yml")
+
+begin
+    CONFIG['sso_auth'] = YAML.load_file(path)
+rescue Exception
+    puts "Error while parsing config file #{path}"
+    CONFIG['sso_auth'] = Hash.new
+end
+
+Rails.application.config.middleware.use OmniAuth::Builder do
+    CONFIG['sso_auth'].each do |name, options|
+        options[:name] = name
+        provider (options['strategy'] || name), options
+    end
+end