This patch fixes the problems below:
1. flavor.unset_keys() is not checked in flavor unit tests.
So check them in the tests.
2. test_flavor_unset_no_project makes no sense. It is OK to
specify ``--project ''`` without raising any exception.
It can pass because in the test, we set nither project
nor property. So remove this test.
Change-Id: I04e537349936343b6d8c85b06bc6d0ba6bd41d6f