enable server ls command to accept the format option

[john801205]

Summary

This PR allows users to specify format for the server ls command.

Usage

$ make build
$ ./bin/rancher server ls -h
List all servers

Usage:
  rancher server ls [OPTIONS]

Options:
   --format value  'json', 'yaml' or Custom format: '{{.Name}} {{.URL}}'
$ ./bin/rancher server ls --format '{{.Name}}'
beta
prod