Call controller methods directly in test_evacuate

In API unit testing, making call through WSGI is little bit overhead,
wherever applicable, unit tests can make direct call to controller
methods.

Some unit tests needs WSGI call to tests more than one plugin together.

This patch makes above cleanup in test_evacuate.

Partially implements blueprint v2-on-v3-api

Change-Id: Ia664a9c71d7d62f39a2dbed7605d102526f4520c
Merge "Call controller methods directly in test_evacuate"