Add option to report build page

This adds a tenant option to use the Zuul web build page as the
URL reported to the code review system when a build completes.

The setting is per-tenant (because it requires that the tenant
have a working SQL reporter configured in all pipelines) and
defaults to false, since we can't guarantee that.  In the future,
we expect to make SQL reporting implicit, then this can default
to true and eventually be deprecated.

A new zuul.conf option is added and marked required to supply
the root web URL.  As we perform further integration with the web
app, we may be able to deprecate other similar settings, such
as "status_url".

Change-Id: Iaa3be10525994722d020d2aa5a7dcf141f2404d9

tests/fixtures/config/build-page/git/common-config/playbooks/python27.yaml

@@ -0,0 +1,2 @@
+- hosts: all
+  tasks: []

tests/fixtures/config/build-page/git/common-config/zuul.yaml

@@ -0,0 +1,24 @@
+- pipeline:
+    name: check
+    manager: independent
+    trigger:
+      gerrit:
+        - event: patchset-created
+    success:
+      gerrit:
+        Verified: 1
+    failure:
+      gerrit:
+        Verified: -1
+
+- job:
+    name: base
+    parent: null
+
+- job:
+    name: python27
+    nodeset:
+      nodes:
+        - name: controller
+          label: ubuntu-trusty
+    run: playbooks/python27.yaml

tests/fixtures/config/build-page/git/org_project1/README

@@ -0,0 +1 @@
+test

tests/fixtures/config/build-page/git/org_project1/zuul.yaml

@@ -0,0 +1,4 @@
+- project:
+    check:
+      jobs:
+        - python27

tests/fixtures/config/build-page/git/org_project2/README

@@ -0,0 +1 @@
+test

tests/fixtures/config/build-page/git/org_project2/zuul.yaml

@@ -0,0 +1,4 @@
+- project:
+    check:
+      jobs:
+        - python27

tests/fixtures/config/build-page/git/org_project3/README

@@ -0,0 +1 @@
+test

tests/fixtures/config/build-page/git/org_project3/zuul.yaml

@@ -0,0 +1,4 @@
+- project:
+    check:
+      jobs:
+        - python27