Better configuration options for setting rest api url in jberet-ui

Currently users need to modify the restApiUrl variable in batchRestService.js.

Need to make it easier to configure this, for instance,

• take from gulp command line args --restUrl xxx
• add a configuration file, config.json, to configure restUrl;
• take from environment variable env.JBERET_REST_URL;

Then add a gulp task to replace the token in batchRestService.js with the user configured value.