Refactor buildpack variable
Refactor buildpack variable

Moved it into single variable and made everything in the test use static
version

[#168848842](https://www.pivotaltracker.com/story/show/168848842)

Co-authored-by: Brendan Smith <8591d66a77fc00d117843ef4d4bc1fbb80fd377f@pivotal.io>

cloudfoundry/cli,cloudfoundry/cli,cloudfoundry/cli