Since OpenStack Train release the 'cpu_models' config option
has superseded the 'cpu_model' config option in the nova.conf.
This patch adds support for the new 'cpu_models' allowing a user
to provide a comma separated list of supported, named CPU models.
This patch also includes a unit test for the cpu_mode='custom'.
Closes-bug: #2025914
Change-Id: I30328abc07d3304f1bfb67c81360fb5229214c97