Assignment to reserved built-in symbol: filter

In keystone/common/controller.py, filterprotected::_filterprotected(),
built-in symbol filter was re-assignment as local vars, it is easily
lead to a misunderstanding, so rename them.
This patch do not modify any functionality.

Change-Id: I6e7965b445496c7ad140e8320ded61706d62895e
Remove an enumerate call

Refactor keystone/tests/test_backend.py, remove a useless enumerate
call in for loop.

Change-Id: Ib0297ff0b2d1e0a6dac5821523a93929e4608601
Merge "Remove an enumerate call"
Merge "Assignment to reserved built-in symbol: filter"