Add private docker registry support to update-intergration-configs task

[aminjam]

What is this change about?

CATS has the option to test private docker registries, this PR adds that support to update-integration-configs task so that the credentials doesn't have to get committed to repositories if it's public.

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

How should this change be described in release notes?

Add support for populating integration-configs with private docker registries

What is the level of urgency for publishing this change?

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

[cf-gitbot]

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

https://www.pivotaltracker.com/story/show/173951809

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