Add zuul:value sphinx directive

The keywords 'independent' and 'dependent' for pipeline.manager
shouldn't actually have the yaml path prefix on them -- they are
literal values which should appear in the pipeline.manager dict.
It suggested that the syntax was:

  pipeline:
    manager: pipeline.manager.dependent

rather than the correct:

  pipeline:
    manager: dependent

Change-Id: I7885f5769c2faabc9ea845493e6bab9835e87b8c

doc/source/user/config.rst

@@ -140,7 +140,7 @@ optional unless otherwise specified):
 
    .. _independent_pipeline_manager:
 
-   .. zuul:attr:: independent
+   .. zuul:value:: independent
 
      Every event in this pipeline should be treated as independent of
      other events in the pipeline.  This is appropriate when the order
@@ -159,7 +159,7 @@ optional unless otherwise specified):
 
    .. _dependent_pipeline_manager:
 
-   .. zuul:attr:: dependent
+   .. zuul:value:: dependent
 
      The dependent pipeline manager is designed for gating.  It
      ensures that every change is tested exactly as it is going to be

zuul/sphinx/zuul.py

@@ -75,6 +75,26 @@ class ZuulAttrDirective(ZuulConfigObject):
                                           targetname, '', None))
 
 
+class ZuulValueDirective(ZuulConfigObject):
+    has_content = True
+
+    def handle_signature(self, sig, signode):
+        signode += addnodes.desc_name(sig, sig)
+        return sig
+
+    def add_target_and_index(self, name, sig, signode):
+        targetname = self.objtype + '-' + name
+        if targetname not in self.state.document.ids:
+            signode['names'].append(targetname)
+            signode['ids'].append(targetname)
+            signode['first'] = (not self.names)
+            self.state.document.note_explicit_target(signode)
+
+        indextext = '%s (%s)' % (name, self.objtype)
+        self.indexnode['entries'].append(('single', indextext,
+                                          targetname, '', None))
+
+
 class ZuulDomain(Domain):
     name = 'zuul'
     label = 'Zuul'
@@ -82,11 +102,13 @@ class ZuulDomain(Domain):
     object_types = {
         'configobject': ObjType('configobject'),
         'attr': ObjType('attr'),
+        'value': ObjType('value'),
     }
 
     directives = {
         'configobject': ZuulConfigobjectDirective,
         'attr': ZuulAttrDirective,
+        'value': ZuulValueDirective,
     }