Catch wrong exception due to oslo's change.

[Ya Hong Du]

In neutron/agent/linux/ip_lib.py, if option 'ip_lib_force_root' is not registered, 'NoSuchOptError' will be catched.
But now, oslo.config changed.
According to below commit, '__getattr__' will raise 'AttributeError' if any exception happens.
https://review.openstack.org/#/c/37276/

We didn't handle 'AttributeError' in above file so that neutron-dhcp-agent can't start successfully.

[Mark McClain (markmcclain)]

The fix should work with latest and the previous release of the config library

[ZhiQiang Fan (aji-zqfan)]

according to the oslo source code, i notice that the NoSuchOptError inherits from AttributeError (actually is multiple inherit). So i think just catch AttrbuteError will be fine, however, i will read more carefully and test it.

[Kyle Mestery (mestery)]

This fixed this for me, looks to be an oslo dependency issue of some sort:

cd /usr/local/lib/python2.7/dist-packages
sudo rm -rf oslo*
sudo pip install --upgrade http://tarballs.openstack.org/oslo.config/oslo.config-1.2.0a3.tar.gz#egg=oslo.config-1.2.0a3

[Matt Riedemann (mriedem)]

Looks like this should fix the problem:

https://github.com/openstack/oslo.config/commit/c85c8e6c4d5a921aa36a4774fe6bd8ab07d58fb3