Add test case for config generator hostname check

When I fixed the referenced bug I failed to add a unit test to
cover the error case, so it's been up to the reviewers to make sure
it doesn't regress.  Let's give the reviewers a break and add a
proper test case.

Change-Id: I46c0f082acd9da6e84c7ac4d4cba387ec97c23c8
Related-Bug: #1205411
Merge "Add test case for config generator hostname check"