Clean up the webkitpy.layout_tests.servers.lighttpd implementation

to go along with all the other server code refactoring.

R=scottmg@chromium.org
BUG=30536

Review URL: https://codereview.chromium.org/157803012

git-svn-id: svn://svn.chromium.org/blink/trunk@167549 bbb929c8-8fbe-4397-9dbb-9b2b20218538