POC: increase RAM 6->8GB for compute nodes
POC: increase RAM 6->8GB for compute nodes

With current concurrency of tempest testing being
8 (default) some jobs show random tempest tests
fail occasionally with 'no valid host found'.
After investigating a few examples of such issue
(phase2 osp16.1 .*1comp ones) it seems that
there isn't enough RAM to run 8 tempest tests
(each of them spawning up to 4 cirros VMs,
flavor 128MB).

RHOSINFRA-3708

Change-Id: If18882dd71976d1666447800365d15034589cd7b

redhat-openstack/infrared,redhat-openstack/infrared,redhat-openstack/infrared