Add instance flag to run-errand task

[davewalter]

What is this change about?

This PR adds an INSTANCE parameter to the run-errand task. It is required to allow CI jobs to disambiguate between different-configured copies of the same errand job within a deployment. For the Diego Persistence team, we test our nfsbrokerpush errand with different configuration for the backing store used by the NFS broker application. This change allows us to specify which instance of the errand should be run.

Please provide contextual information.

#161974894

Please check all that apply for this PR:

• [ ] introduces a new task
• [x] changes an existing task
• [ ] changes the Dockerfile
• [ ] introduces a breaking change (other users will need to make manual changes when this is released)

Did you update the README as appropriate for this change?

• [ ] YES
• [x] N/A - the run-errand task is not referenced in the README

How should this change be described in release notes?

Updated Tasks:

• run-errand
  • Added parameterized support for specifying the instance that should be used to run an errand.

What is the level of urgency for publishing this change?

• [ ] Urgent - unblocks current or future work
• [x] Slightly Less than Urgent

Tag your pair, your PM, and/or team!

@paulcwarren @juluan-hj

[cfdreddbot]

:white_check_mark: Hey davewalter! The commit authors and yourself have already signed the CLA.

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/164183914

The labels on this github issue will be updated when the story is started.