Ignore updates of buildpack (e.g. second cf push)

[kramerul]

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

• A short explanation of the proposed change:

When running a cf push twice with a manifest.yml which includes a buildpacks setting, the second push fails with undefined method 'buildpacks='

• An explanation of the use cases your change solves

This also relates to cloudfoundry/cf-for-k8s#88 which was closed.
The current behaviour is not consitent. The first push succeeds and the second fails. It should either fail in both cases or succeed in both cases. We would vote for the later.

• Links to any other associated PRs

• I have reviewed the contributing guide

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

• I have made this pull request to the master branch

• I have run all the unit tests using bundle exec rake

• I have run CF Acceptance Tests
  Not require for this simple change.

[cf-gitbot]

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

https://www.pivotaltracker.com/story/show/172695119

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

[sethboyles]

Also fails with cf push appname -b buildpack. It's interesting that we accept buildpack arguments at all when they do nothing, but I agree we should at least be consistent. Thanks for the PR!