Don't traceback in the API on invalid keypair.

When trying to get a keypair that did not exist, a traceback would end
up in the nova-api log.  Handle this exception in the API extension so
that it doesn't make a mess of the log.

Also add a unit test for this particular scenario.

Fix bug 1145545.

Change-Id: Id1647d4ac56629c2931099320d271e34fc2f99b9
Merge "Don't traceback in the API on invalid keypair."