Add value format for list command

The value format for the show command is very handy for shell
scripting and a value format for the list command would be just
as handy.  It will allow command substitution and piping like:

    os container list --format value | grep tmp | while read NAME
    do
      os container delete $NAME
    done

Change-Id: If9784c27d689073e9145b2cb8077ac604025a4c3

cliff/formatters/value.py

@@ -1,14 +1,22 @@
 """Output formatters values only
 """
 
+import six
+
+from .base import ListFormatter
 from .base import SingleFormatter
 
 
-class ValueFormatter(SingleFormatter):
+class ValueFormatter(ListFormatter, SingleFormatter):
 
     def add_argument_group(self, parser):
         pass
 
+    def emit_list(self, column_names, data, stdout, parsed_args):
+        for row in data:
+            stdout.write(' '.join(map(six.text_type, row)) + u'\n')
+        return
+
     def emit_one(self, column_names, data, stdout, parsed_args):
         for value in data:
             stdout.write('%s\n' % str(value))

cliff/tests/test_formatters_value.py

@@ -13,3 +13,16 @@ def test_value_formatter():
     sf.emit_one(c, d, output, None)
     actual = output.getvalue()
     assert expected == actual
+
+
+def test_value_list_formatter():
+    sf = value.ValueFormatter()
+    c = ('a', 'b', 'c')
+    d1 = ('A', 'B', 'C')
+    d2 = ('D', 'E', 'F')
+    data = [d1, d2]
+    expected = 'A B C\nD E F\n'
+    output = StringIO()
+    sf.emit_list(c, data, output, None)
+    actual = output.getvalue()
+    assert expected == actual

setup.cfg

@@ -29,6 +29,7 @@ packages =
 cliff.formatter.list =
     table = cliff.formatters.table:TableFormatter
     csv = cliff.formatters.commaseparated:CSVLister
+    value = cliff.formatters.value:ValueFormatter
 
 cliff.formatter.show =
     table = cliff.formatters.table:TableFormatter