Install Tmux and GNU Screen for TripleO Devtest

Devtest script takes a long time to finish, so we often use GNU Screen
or Tmux to run it, so that we don't have to be connected to the lab
machine all the time. Let's install them both so that users can use
their favorite one.

Change-Id: Iee8e511a020ba82e3cfb7e5307d536cc8f656984