Add tcpdump to our required package list in kvm-admin.

For now, we have to have tcpdump to do our automatic switch
configuration.  Until I pull this out of ohai and put it in its own
role, it needs to be in place before we tryt and run a kvm admin node.
Merge pull request #172 from VictorLowther/require-tcpdump

+2 Add tcpdump to our required package list in kvm-admin.