config: allow env.yaml to contain configuration in a single file

- add `SingleFileScope` to configuration, which allows us to pull config
  sections from a single file.

- update `env.yaml` and tests to ensure that the env.yaml schema works
  when pulling configurtion from the env file.

lib/spack/llnl/util/lang.py

@@ -146,6 +146,26 @@ def has_method(cls, name):
     return False
 
 
+def union_dicts(*dicts):
+    """Use update() to combine all dicts into one.
+
+    This builds a new dictionary, into which we ``update()`` each element
+    of ``dicts`` in order.  Items from later dictionaries will override
+    items from earlier dictionaries.
+
+    Args:
+        dicts (list): list of dictionaries
+
+    Return: (dict): a merged dictionary containing combined keys and
+        values from ``dicts``.
+
+    """
+    result = {}
+    for d in dicts:
+        result.update(d)
+    return result
+
+
 class memoized(object):
     """Decorator that caches the results of a function, storing them
        in an attribute of that function."""

lib/spack/llnl/util/tty/colify.py

@@ -10,7 +10,7 @@
 
 import os
 import sys
-from six import StringIO
+from six import StringIO, text_type
 
 from llnl.util.tty import terminal_size
 from llnl.util.tty.color import clen, cextra
@@ -137,7 +137,7 @@ def colify(elts, **options):
             % next(options.iterkeys()))
 
     # elts needs to be an array of strings so we can count the elements
-    elts = [str(elt) for elt in elts]
+    elts = [text_type(elt) for elt in elts]
     if not elts:
         return (0, ())