Use 4c-16g builder for tox-verify jobs

Use 4c-16g builder for tox-verify jobs

The TransportPCE project does not appear to require 8 CPU system
to successfully build the Maven project. Looking at the Sonar and
CLM jobs this project builds just fine with the default
centos7-builder-4c-4g system.

TransportPCE does require 8g system for the tox-verify job though
due to it's use of Docker. There is no 4c-8g system available by
Vexxhost but there is a 4c-16g system which is about 33% cheaper
than the 8c-8g flavor.

Change-Id: Ib9c4b11749aec5d6eddef08bd8a708e858aaac50
Signed-off-by: Thanh Ha <zxiiro@gmail.com>