Update resource manager for fixed stevedore issue

Stevedore commit 69e17801a3f0cf37257069da7dc83e2608f31aa5 fixed a
known issue in conductor.resource_manager.

This patch updates the Ironic code to use stevedore's new accessor.

Change-Id: I0f57831a70368afb5ea321c0e607f762d464c2a0