Export location of stacks.yml for migrations

[idoru]

Signed-off-by: Dave Goddard dave@goddard.id.au

Thanks for contributing to the capi_release. To speed up the process of reviewing your pull request please provide us with:

• A short explanation of the proposed change:
  Provide an environment variable during database migrations which exposes the location of the stacks.yml config file.

• An explanation of the use cases your change solves
  A migration which needs to alter data to include the default stack in some situations has no reliable way of identifying the default stack. If, however, the location of the CC's stacks.yml file is known, a migration can read it to find out the default stack.

• Links to any other associated PRs
  Used to support migration in the following cloud_controller_ng PR

• I have viewed signed and have submitted the Contributor License Agreement

• I have made this pull request to the develop branch

• I have run CF Acceptance Tests on bosh lite

[cfdreddbot]

Hey idoru!

Thanks for submitting this pull request! I'm here to inform the recipients of the pull request that you and the commit authors have already signed the CLA.

[cf-gitbot]

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

https://www.pivotaltracker.com/story/show/154527476

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

[ericpromislow]

Nothing is currently using STACKS_YML. I'm wondering if we should delay merging this in until something else is using it.

[idoru]

@ericpromislow yes, the first use case is in the cloud_controller_ng PR so merging this should probably be co-ordinated with that.