Make the git web url a template

We have deployed our Gerrit with cgit so the old gitweb urls provided by
Zuul's gerrit connection no longer work. Add in a new config option on
Gerrit connections to specify a url template string which we can modify
to point at our cgit instance. This should in theory also support github
users too.

The default is to continue pointing at Gerrit's built in gitweb
instance.

Change-Id: I91d77e309cfeea0e90a85f926aca9b8c347b0385

tests/fixtures/zuul-connections-gitweb.conf

@@ -0,0 +1,26 @@
+[gearman]
+server=127.0.0.1
+
+[scheduler]
+tenant_config=main.yaml
+
+[merger]
+git_dir=/tmp/zuul-test/merger-git
+git_user_email=zuul@example.com
+git_user_name=zuul
+
+[executor]
+git_dir=/tmp/zuul-test/executor-git
+
+[connection gerrit]
+driver=gerrit
+server=review.example.com
+user=jenkins
+sshkey=none
+
+[connection outgoing_smtp]
+driver=smtp
+server=localhost
+port=25
+default_from=zuul@example.com
+default_to=you@example.com