Fixes DiskInfoPollster AttributeError exception
Fixes DiskInfoPollster AttributeError exception

If a NotImplementedError exception is raised while getting samples through
a DiskInfoPollster, another exception occurs, as it tries to access an
attribute that does not exist.

Replaces the usage of manager.inspector with self.inspector.

Change-Id: Ie1bb6eaffc0b2671f8fefb4771dbd2191eb5cf5f
Closes-Bug: #1460207

fabian4/ceilometer,redhat-openstack/ceilometer,sileht/aodh,r-mibu/ceilometer,maestro-hybrid-cloud/ceilometer,eayunstack/ceilometer,chungg/aodh,fabian4/ceilometer,r-mibu/ceilometer,cernops/ceilometer,mathslinux/ceilometer,ityaptin/ceilometer,cernops/ceilometer,openstack/aodh,openstack/aodh,idegtiarov/ceilometer,chungg/aodh,openstack/ceilometer,openstack/ceilometer,isyippee/ceilometer,idegtiarov/ceilometer,sileht/aodh,maestro-hybrid-cloud/ceilometer,pkilambi/ceilometer,ityaptin/ceilometer,pczerkas/aodh,pczerkas/aodh,redhat-openstack/ceilometer,eayunstack/ceilometer,isyippee/ceilometer,pkilambi/ceilometer,mathslinux/ceilometer