Closes-Bug: #1506700
This is the 4th change list of a series of changes that will eventually
convert chef installer as a plugin.
It moves chef installer related tests to plugins directory
extract a method get_plugins_config_files from load_config in util.py,
this function might be used directly, add a unit test for this function
Change-Id: Idbd54ce11ab4489b356b402d176c9df2f688acc2