use role name rather than token displayname in generated mysql usernames

If a single token generates multiple myself roles, the generated mysql
username was previously prepended with the displayname of the vault
user; this makes the output of `show processlist` in mysql potentially
difficult to correlate with the roles actually in use without cross-
checking against the vault audit log.

See https://github.com/hashicorp/vault/pull/1603 for further discussion.

builtin/logical/mysql/path_role_create.go

@@ -51,7 +51,7 @@ func (b *backend) pathRoleCreateRead(
 	}
 
 	// Generate our username and password. MySQL limits user to 16 characters
-	displayName := req.DisplayName
+	displayName := name
 	if len(displayName) > 10 {
 		displayName = displayName[:10]
 	}