Refactor webkitpy.layout_tests.servers.server_base.

This moves some code from servers.lighttpd to server_base so that it can
be reused by the other server classes. This is a prelude to cleaning up
code in the other server classes.

R=scottmg@chromium.org
BUG=30536

Review URL: https://codereview.chromium.org/159723008

git-svn-id: svn://svn.chromium.org/blink/trunk@167051 bbb929c8-8fbe-4397-9dbb-9b2b20218538