mininet crashes with option --host rt

[Cody Burkard]

mininet is missing the dependency cgroup-bin. Without cgroup-bin, the option --host rt will crash.

Output of crash:

mininet@ubuntu-1204:~$ sudo mn --host rt
*** Creating network
*** Adding controller
*** Adding hosts:
Traceback (most recent call last):
  File "/usr/bin/mn", line 280, in <module>
    MininetRunner()
  File "/usr/bin/mn", line 113, in __init__
    self.begin()
  File "/usr/bin/mn", line 249, in begin
    listenPort=listenPort )
  File "/usr/lib/python2.7/dist-packages/mininet/net.py", line 163, in __init__
    self.build()
  File "/usr/lib/python2.7/dist-packages/mininet/net.py", line 315, in build
    self.buildFromTopo( self.topo )
  File "/usr/lib/python2.7/dist-packages/mininet/net.py", line 289, in buildFromTopo
    self.addHost( hostName, **topo.nodeInfo( hostName ) )
  File "/usr/lib/python2.7/dist-packages/mininet/net.py", line 185, in addHost
    h = cls( name, **defaults )
  File "/usr/lib/python2.7/dist-packages/mininet/util.py", line 443, in customized
    return constructor( name, *args, **params )
  File "/usr/lib/python2.7/dist-packages/mininet/util.py", line 406, in customized
    return cls( *args, **kwargs )
  File "/usr/lib/python2.7/dist-packages/mininet/node.py", line 564, in __init__
    CPULimitedHost.init()
  File "/usr/lib/python2.7/dist-packages/mininet/node.py", line 712, in init
    mountCgroups()
  File "/usr/lib/python2.7/dist-packages/mininet/util.py", line 367, in mountCgroups
    raise Exception( "cgroups not mounted on " + cgdir )
Exception: cgroups not mounted on /sys/fs/cgroup

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: mininet 2.0.0-0ubuntu1~ubuntu12.04.1
ProcVersionSignature: Ubuntu 3.13.0-32.57~precise1-generic 3.13.11.4
Uname: Linux 3.13.0-32-generic x86_64
ApportVersion: 2.0.1-0ubuntu17.6
Architecture: amd64
Date: Fri Sep 5 12:38:14 2014
InstallationMedia: Ubuntu 12.04.5 LTS "Precise Pangolin" - Release amd64 (20140807.1)
MarkForUpload: True
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: mininet
UpgradeStatus: No upgrade log present (probably fresh install)