make createtime and lastmodifiedtime for logfileproperties to readonly

Author: QingqingYuan2

specification/mysql/resource-manager/Microsoft.DBforMySQL/preview/2017-12-01-preview/mysql.json

@@ -1508,11 +1508,13 @@
         },
         "createdTime": {
           "type": "string",
+          "readOnly": true,
           "format": "date-time",
           "description": "Creation timestamp of the log file."
         },
         "lastModifiedTime": {
           "type": "string",
+          "readOnly": true,
           "format": "date-time",
           "description": "Last modified timestamp of the log file."
         },

specification/postgresql/resource-manager/Microsoft.DBforPostgreSQL/preview/2017-12-01-preview/postgresql.json

@@ -1507,11 +1507,13 @@
         },
         "createdTime": {
           "type": "string",
+          "readOnly": true,
           "format": "date-time",
           "description": "Creation timestamp of the log file."
         },
         "lastModifiedTime": {
           "type": "string",
+          "readOnly": true,
           "format": "date-time",
           "description": "Last modified timestamp of the log file."
         },