Share common test settings in test_flavor_manage

In PrivateFlavorManageTest, there are duplicated test setting code in each
test. This patch moves these settings into setUp() method to reduce code.
In addition, this patch adds an internal method also for the same purpose.

Change-Id: I31883497174141ac4034922981e0a846f1fc2deb
Merge "Share common test settings in test_flavor_manage"