Remove unused exception "InstanceNotFound"
Remove unused exception "InstanceNotFound"

In nova/virt/docker/driver.py, the "InstanceNotFound" import from
nova.exception. But the "InstanceNotFound" from zun.common.exception
is not used anywhere,so it should be removed.

Change-Id: Ie4d70adf763a6454183a500d427aa44743767501
Partial-Bug: #1651257

kevin-zhaoshuai/zun,kevin-zhaoshuai/zun,kevin-zhaoshuai/zun