Don't modify columns_to_join formal parameter in _manual_join_columns

The _manual_join_columns method was modifying the columns_to_join list
passed to it which, if not copied by the original caller, can introduce
bugs in the calling code if the original columns_to_join list is re-used
later, e.g. making nova objects with an expected_attrs list that has
been mutated by the DB API call.

This change makes a copy of the original list and passes back the new
list for the DB API methods to use, and adds some documentation to
hopefully clarify intent and usage.

A test is added to demonstrate the expected behavior of the utility
method.

Closes-Bug: #1393491

Change-Id: I49511f608291b66642ced00a3c13833e18fab3d7
Merge "Don't modify columns_to_join formal parameter in _manual_join_columns"