Move back to 8G tripleo nodes

This was increased some time ago to workaround a bug, the bug is now
fixed. max-servers on the rh1 provider can also be increased to 18(we
have 18 testenvs) as we will now be able to hold more instances.

Change-Id: I23e28b7cf55b099df73f55bf57d7cc0cbc78d969