fix fake_buildconfigs methods

[guilhermebr]

This PR fixes the follow error when testing buildconfig Instantiate and InstantiateBinary methods:

error:

panic: interface conversion: runtime.Object is *api.BuildConfig, not *api.Build [recovered]
    panic: interface conversion: runtime.Object is *api.BuildConfig, not *api.Build

[bparees]

out of curiosity is there a test that's hitting this error, or were you trying to use it and encountered this? none of our tests are failing so i'm trying to understand how this came up.

[bparees]

[test]

[openshift-bot]

Evaluated for origin test up to 762889c

[guilhermebr]

@bparees I found this error in a new test I'm writing, I just opened separated because I don't have sure when I will finish this new test, and it have many other changes

[guilhermebr]

@bparees if you want I can open the PR as WIP (without these change) for you see the test failing

[openshift-bot]

continuous-integration/openshift-jenkins/test SUCCESS (https://ci.openshift.redhat.com/jenkins/job/test_pr_origin/9541/)

[bparees]

@guilhermebr that's fine, i'll merge it, thanks!
[merge]

[openshift-bot]

Evaluated for origin merge up to 762889c

[openshift-bot]

continuous-integration/openshift-jenkins/merge SUCCESS (https://ci.openshift.redhat.com/jenkins/job/test_pr_origin/9548/) (Image: devenv-rhel7_5111)