switch-softwaretype: make it possible to call it in chain

Store only first 2 system arguments in 'args' so that to facilitate use of switch-softwaretype recipe in chain.
For example:-
In one buildout having section with recipe switch-softwaretype and running buildout for a file which in succession is calling another buildout, so that we don't have a conflict between other system arguments.

slapos/recipe/switch_softwaretype.py

@@ -40,7 +40,10 @@ class Recipe:
   def install(self):
     # XXX-Antoine: We gotta find a better way to do this. I tried to check
     # out how slapgrid-cp was running buildout. But it is worse than that.
-    args = sys.argv[:]
+
+    # Using first two arguments to prevent the conflict that might arise while
+    # calling recipe in succession. 
+    args = sys.argv[:2]
     for x in self.buildout["slap-connection"].iteritems():
       args.append("slap-connection:%s=%s" % x)
     for x in "directory", "eggs-directory", "develop-eggs-directory":