Update Mininet VagrantBox (part 2)

    Add xauth pkg to mininet
    Add file: resources/mininet/cheatsheet.txt
    Rename topology inside topo.py to L3TestTopo (instead of MyTopo)

Change-Id: Ia5b7ffe59a29cc1fef00bf77faac63bf27669343
Signed-off-by: Flavio Fernandes <ffernand@redhat.com>
Merge "    Update Mininet VagrantBox (part 2)"