Add chef-client-is-enabled script to the chef-client-disabler

Per discussion here, in addition to intercepting calls to chef-client when it's disabled, we would also like to be able to easily check whether or not it is disabled.

Add a chef-client-is-enabled script to the gitlab-server::chef-client-disabler recipe. Expected behavior:

• If chef-client is enabled, this script will say so and exit 0.
• If chef-client is disabled, this script will exit non-zero and show when, by whom, and for what reason (i.e. same output as though chef-client had been run and intercepted).