extract git_checkout into common function

we were basically doing the same thing in three places, make this
a common function so we stop repeating ourselves.

Change-Id: I2bc13a95ce1fbd4fd2645c6e8cc37823b98a6190
move single host setup activities to setup_host

we previously were doing corrections to the host, like the hostname
and vdb setup in setup_workspace instead of setup_host. These really
should only be done once per setup environment, and thus should
not be in setup_workspace.

Change-Id: I53853954f7712a04a205cd3e0b87ece3465e1e0b