fixup! Fix random test failure with make flags on test node

```
======================================================================
ERROR: /srv/slapgrid/slappart0/parts/slapos.recipe.cmmi/slapos/recipe/cmmi/README.txt
Doctest: README.txt
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/opt/slapgrid/298b8e2a56fa3803aa68aa4cfe6a3b96/parts/python2.7/lib/python2.7/doctest.py", line 2194, in setUp
    self._dt_setUp(test)
  File "/srv/slapgrid/slappart0/parts/slapos.recipe.cmmi/slapos/recipe/cmmi/tests.py", line 26, in setUp
    os.environ.pop('MAKEFLAGS')
  File "/opt/slapgrid/298b8e2a56fa3803aa68aa4cfe6a3b96/parts/python2.7/lib/python2.7/os.py", line 505, in pop
    return self.data.pop(key, *args)
KeyError: 'MAKEFLAGS'
```

slapos/recipe/cmmi/tests.py

@@ -23,7 +23,7 @@ optionflags = (doctest.ELLIPSIS |
 
 
 def setUp(test):
-    os.environ.pop('MAKEFLAGS')
+    os.environ.pop('MAKEFLAGS', None)
     zc.buildout.testing.buildoutSetUp(test)
     zc.buildout.testing.install('slapos.recipe.build', test)
     zc.buildout.testing.install_develop('slapos.recipe.cmmi', test)