Rename "filter" parameters

The word "filter" doesn't help very much with clarity and it's
sometimes confusing whether you need to use "username" or
"username_filter".  Just drop the filter suffix where it's used.

Tests are intentionally not altered in this change to verify
backwords compatibility.

Change-Id: I2edddad2e1f6ce4c86265de5f7a8cda1ebf436df