Configure /etc/hosts via os-collect-config script

Configure /etc/hosts via os-collect-config script

This patch moves the t-i-e element code for hosts configuration
into a t-h-t shell script that gets driven by a os-collect-config
script hook.

This helps accomplish several goals:

 - moves us away from t-i-e

 - gives us better signal handling in the error case (where the
   previous element relied on 99-refresh-completed

 - Allows the t-h-t undercloud installer to more easily consume this
   since it doesn't rely on the old os-apply-config metadata (which
   that installer doesn't support).

Change-Id: I73c3d4818ef531a3559fab272521f44519e2f486