[authn] Added the Labels property to the TokenDBValue struct (cesanta#217)

This patch extends the `TokenDBValue` struct in order to allow storing "labels" associated
with a user directly in the data structure, ie. without a need for secondary storage.

The primary motivation is related to the possibility of storing user's Github teams
as `labels`, and using them in the ACL configuration.

Author: karmi

auth_server/authn/tokendb.go

@@ -70,6 +70,7 @@ type TokenDBValue struct {
 	// DockerPassword is the temporary password we use to authenticate Docker users.
 	// Generated at the time of token creation, stored here as a BCrypt hash.
 	DockerPassword string `json:"docker_password,omitempty"`
+	Labels         Labels `json:"labels,omitempty"`
 }
 
 // NewTokenDB returns a new TokenDB structure