Add github reporter status-url config option

Make it possible to set the github status link to something other than the
zuul server status_url. This allows the github status to link to the log output
for the jobs. If set, this url will be run through the same formatter
as the url_pattern used for comment links. If unset, it will fall back to the
previous behaviour of using the zuul server status_url.

Handle status-url from the reporter config and the zuul config the same
way and remove the hard coded change url format from zuul.conf. If a
user wants to specify the change id in the status url they should use
a template string in the same way as configuring a reporter.

Co-Authored-By: Jamie Lennox <jamielennox@gmail.com>
Change-Id: I5f2ff35bb38426ddcd7f65798b4f114256a54847
diff --git a/tests/fixtures/zuul-github-driver.conf b/tests/fixtures/zuul-github-driver.conf
index ab34619..dfa813d 100644
--- a/tests/fixtures/zuul-github-driver.conf
+++ b/tests/fixtures/zuul-github-driver.conf
@@ -3,7 +3,7 @@
 
 [zuul]
 job_name_in_report=true
-status_url=http://zuul.example.com/status
+status_url=http://zuul.example.com/status/#{change.number},{change.patchset}
 
 [merger]
 git_dir=/tmp/zuul-test/git