only remove network states from Nova when n-net is enabled

When n-net is not enabled but q-dhcp is enabled then it's not
necessary to remove the states directory of nova-network.

Change-Id: Ib4b37aa7df2b3cda3048b278ddad24e87e3744c9
Merge "only remove network states from Nova when n-net is enabled"
Merge branch 'master' of github.com:openstack-dev/devstack