bugfix: fix config merge order for OrderdDicts

The logic in `config.py` merges lists correctly so that list elements
from higher-precedence config files come first, but the way we merge
`dict` elements reverses the precedence.

Since `mirrors.yaml` relies on `OrderedDict` for precedence, this bug
causes mirrors in lower-precedence config scopes to be checked before
higher-precedence scopes.

We should probably convert `mirrors.yaml` to use a list at some point,
but in the meantie here's a fix for `OrderedDict`.

- [x] ensuring that keys are ordered correctly in `OrderedDict` by
  re-inserting keys from the destination `dict` after adding the keys from
  the source `dict`.

Assuming a default spack configuration, if we run this:

```console
$ spack mirror add foo https://bar.com
```

Results before this change:

```console
$ spack config blame mirrors
---                                                          mirrors:
/Users/gamblin2/src/spack/etc/spack/defaults/mirrors.yaml:2    spack-public: https://spack-llnl-mirror.s3-us-west-2.amazonaws.com/
/Users/gamblin2/.spack/mirrors.yaml:2                          foo: https://bar.com
```

Results after:

```console
$ spack config blame mirrors
---                                                          mirrors:
/Users/gamblin2/.spack/mirrors.yaml:2                          foo: https://bar.com
/Users/gamblin2/src/spack/etc/spack/defaults/mirrors.yaml:2    spack-public: https://spack-llnl-mirror.s3-us-west-2.amazonaws.com/
```

lib/spack/spack/config.py

@@ -944,6 +944,10 @@ def they_are(t):
         # track keys for marking
         key_marks = {}
 
+        # track this to ensure that source items come *before* dest in
+        # iteration order with OrderdDicts
+        dest_keys = [dk for dk in dest.keys() if dk not in source]
+
         for sk, sv in iteritems(source):
             if _override(sk) or sk not in dest:
                 # if sk ended with ::, or if it's new, completely override
@@ -958,6 +962,12 @@ def they_are(t):
             # to copy mark information on source keys to dest.
             key_marks[sk] = sk
 
+        # ensure that dest keys come after source
+        for dk in dest_keys:
+            value = dest[dk]
+            del dest[dk]
+            dest[dk] = value
+
         # ensure that keys are marked in the destination. The
         # key_marks dict ensures we can get the actual source key
         # objects from dest keys