adjust default cpu and ram allocation ratios

As per support discussion, modify:

nova_cpu_allocation_ratio = 2.0
nova_ram_allocation_ratio = 1.0

Also add commented entry to user_variables to allow overriding

Conflicts:
	rpc_deployment/inventory/group_vars/nova_all.yml

Change-Id: I5563374a6e950f81e1e013d6e741993c1846ded6