Move root_helper deprecation warning into execute.
Move root_helper deprecation warning into execute.

By moving this warning into the execute function and out of the top
level utils module we eliminate warnings that could occur when
Nova code includes utils.py but doesn't setup a config file properly.

Fixes LP Bug #1032657.

Change-Id: Iae41ead251a64717de3bf7cba3fa206cda278a20

openstack/oslo.versionedobjects,citrix-openstack-build/oslo.versionedobjects