Bump VM minimum requirments to 3G

Increase the amount of memory required for VMs in a virtual tripleo
environment to 3G. We had been getting a lot of failures in CI on the
ubuntu overcloud jobs, various services were running out of memory.

The CI testenvs have already be rebuilt host 3G VMs.

Change-Id: Ia0fd8e3f86f5ca786356a460c91f33cfaf29c528
Merge "Bump VM minimum requirments to 3G"