Ensure neutron configuration files are cleaned by clean.sh

Add the /etc/neutron directory to the list of directories
that are deleted.

Change-Id: Ia560179a3cf121f8d09a0c10c8b68375fdd13be7
Merge "Ensure neutron configuration files are cleaned by clean.sh"
Merge branch 'master' of https://github.com/openstack-dev/devstack