Allow passing mncmd to CPULimitedHost.popen()

This is not not really intended as an external API (and
it currently isn't documented) but this makes it consistent.

Fixes #743

mininet/node.py

@@ -702,8 +702,8 @@ class CPULimitedHost( Host ):
            args: Popen() args, single list, or string
            kwargs: Popen() keyword args"""
         # Tell mnexec to execute command in our cgroup
-        mncmd = [ 'mnexec', '-g', self.name,
-                  '-da', str( self.pid ) ]
+        mncmd = kwargs.pop( 'mncmd', [ 'mnexec', '-g', self.name,
+                                       '-da', str( self.pid ) ] )
         # if our cgroup is not given any cpu time,
         # we cannot assign the RR Scheduler.
         if self.sched == 'rt':