Make user name in GerritChangeSource optional.

The user name is not required and defaults to the .ssh/config
file content.
This is implicit in "Buildbot Team Members" -- we don't decorate each file with a list of entities that may or may not own a particular line.  (There are a few grandfathered exceptions, mostly in contrib, that already had the header information when we tightened this up)

Can you remove this line?