Fix 'simple_flat_node_deletion' test

In the simple cluster configuration it is impossible
to remove a single controller now. So let's delete
a compute node instead.

Change-Id: I902200fbbd7096f4039b18a0a59fc2eab692c0cf
Closes-bug: #1368820
(cherry picked from commit cc76281c797bfc2a447babd23782a94206dcefcb)