Remove broker and related errands
Remove broker and related errands

- Previously we reduced the broker instances to zero but left the
errands
- We now actually remove the instance_groups, as an instance_group with
zero instances still shows up in the output of `bosh errands`.

[#133894727]

Signed-off-by: Difan Zhao <d4b657a67385123ddb690ebdb0d79b9c6cd38927@pivotal.io>

cloudfoundry/cf-mysql-release,cloudfoundry/cf-mysql-release,cloudfoundry/cf-mysql-release,cloudfoundry/cf-mysql-release