Implement a yaml formatter in cliff

This change replaces the cliff-tablib yaml formatter with an internal
replacement. It differs from the tablib formatter in the following ways:
- always outputs with block formatting rather than the PyYAML default
  of deciding based on value content
- emit_one serialises a simple dict where the column name is the key
  and the data item is the value (rather than a list of dicts with
  'Field' and 'Value' keys)
- emit_one preserves column order by printing a single-item dict for
  each column_name

The cliff release which contains this change will need a corresponding
cliff-tablib release which removes the yaml formatter from its setup.py
entry_points.

Change-Id: I691dbab3dee7c5ec28b1083f87ab1f5c051d582b
Related-Bug: #1308744

doc/source/list_commands.rst

@@ -90,10 +90,34 @@ a script.
     Makefile is 5569 bytes
     source is 408 bytes
 
+yaml
+----
+
+The ``yaml`` formatter uses PyYAML_ to produce a YAML sequence of
+mappings.
+
+.. _PyYAML: http://pyyaml.org/
+
+::
+
+    (.venv)$ cliffdemo files -f yaml
+    - Name: dist
+      Size: 4096
+    - Name: cliffdemo.egg-info
+      Size: 4096
+    - Name: README.rst
+      Size: 960
+    - Name: setup.py
+      Size: 1807
+    - Name: build
+      Size: 4096
+    - Name: cliffdemo
+      Size: 4096
+
 Other Formatters
 ----------------
 
-Formatters using tablib_ to produce JSON, YAML, and HTML are available
+Formatters using tablib_ to produce JSON and HTML are available
 as part of `cliff-tablib`_.
 
 .. _cliff-tablib: https://github.com/dreamhost/cliff-tablib

doc/source/show_commands.rst

@@ -83,10 +83,27 @@ value of the field or fields.
     (.venv)$ echo $SIZE
     5916
 
+yaml
+----
+
+The ``yaml`` formatter uses PyYAML_ to produce a YAML mapping where
+the field name is the key.
+
+.. _PyYAML: http://pyyaml.org/
+
+::
+
+    (.venv)$ cliffdemo file -f yaml setup.py
+    Name: setup.py
+    Size: 1807
+    UID: 1000
+    GID: 1000
+    Modified Time: 1393531476.9587486
+
 Other Formatters
 ----------------
 
-Formatters using tablib_ to produce JSON, YAML, and HTML are available
+Formatters using tablib_ to produce JSON and HTML are available
 as part of `cliff-tablib`_.
 
 .. _cliff-tablib: https://github.com/dreamhost/cliff-tablib