Fix invalid raise in test_compute_mgr
Fix invalid raise in test_compute_mgr

This patch replaces that invalid raise with a proper exception
raise.

TrivialFix

See same related fix in c52d34f1b46f88ae2786cfb873f9b12a1a01b629

Change-Id: Ie25c4d78715d1d02a22d051a898a55bc5eea4932
Closes-Bug: #1774556

klmitch/nova,openstack/nova,mikalstill/nova,mahak/nova,gooddata/openstack-nova,mahak/nova,openstack/nova,gooddata/openstack-nova,rahulunair/nova,mikalstill/nova,mikalstill/nova,klmitch/nova,klmitch/nova,mahak/nova,gooddata/openstack-nova,rahulunair/nova,openstack/nova,gooddata/openstack-nova,klmitch/nova,rahulunair/nova