Set yum_update in group_vars/{distro}-{distro_ver}

In order to handle mixed environments cleanly, set the yum_update flag
appropriately in the group_vars to make it easy to switch the yum_update
on and off for an entire group of os-ver combos.

Change-Id: I478e81a7f4f6bd941e89c99bb0b6112c94fc5e33