Update CHEF_SERVER_COOKBOOK_ZIP_URL

[joshuap-cfy]

In https://github.com/cloudify-cosmo/cloudify-chef-plugin/blob/master/system_tests/manager/chef_plugin_test.py

CHEF_SERVER_COOKBOOK_ZIP_URL = (
'https://github.com/opscode-cookbooks/chef-server/archive/'
'c588a4c401d3fac14f70d3285fe49eb4dccd9759.zip'

Is pretty outdated.

chef-server cookbook README
v2.1.2 (2014-03-18)

• [COOK-4386] - 'package_options' attribute added

We could update to
v2.1.6 (2014-05-22)

• COOK-4660 - Adds a OS version and image updates to the testing harness

or even

5.1.0 (2016-09-16)

• Testing updates
• Require Chef 12.1 not 12.0