Fix bad classname

During refactor https://review.openstack.org/52562 the rename of
BaseComputeAdminTest did not happen at
tempest.api.compute.admin.test_host_negative. This patch corrects the
error.

Fixes bug 1244917

Change-Id: Ia6949b512ee878d9f122117a42a1ac73667bde6b
Merge "Fix bad classname"