Revert "Fix use of novaclient.exceptions.NotFound"

It was fixed in nova client now, so, we need to revert this fix too.
https://github.com/openstack/python-novaclient/releases/tag/2.18.1

https://github.com/openstack/python-novaclient/compare/2.18.0...2.18.1

This reverts commit 29026c88a1a0528ca9a73a941cb9d49015d83342.

Change-Id: I79503e4c33959ae43d5f2a4daf0d383f1aecd1b4
Merge "Revert "Fix use of novaclient.exceptions.NotFound""