Fix IOError with gnomekeyring.find_network_password_sync

find_network_password_sync throws a gnomekeyring.IOError
when a non-root user tries to run nova client
from a ssh console. If we don't catch this exception nova client
throws the traceback (shown in the bug report) and stops.
If we catch this exception (just like we catch ValueError),
and return None, Nova client executes just fine.

Fixes LP# 1116302

Change-Id: If6937b3f8eafb1dc55224b2ca2bd0f93ae07f8c6