I accidentially dropped this section of the config file when rebasing
between patchset 3 & 4 of change
Ic1848b98625603bf37c0e3dfaf1d0ef6f2c6ac74
Note this is slightly different in that the release is not specified,
so we take the default centos7 image from d-i-b. This is no different
(at the moment they are the same) but means if upstream d-i-b changes
their release, nodepool follows "silently". We will monitor to see if
this is a feature or a bug.
Change-Id: I0c7738f59d014238934b6448ae7bb591eee1bcbe
11891f735d