fix dhcp bulk reload exceptions
fix dhcp bulk reload exceptions

1886969 - The bulk reload code was written for python2 and caused
an exception running under python3. This change works under python3.

1890027 - There was an additional exception triggered when
deleting networks - reading the network from the cache returned 'None'
and this was not properly checked before use.

Change-Id: I4e546c0e37146b1f34d8b5e6637c407b0c04ad4d
Closes-Bug: 1886969
Closes-Bug: 1890027
Signed-off-by: Matt Vinall <7c03863de81724a87fab4bef157ac3e2c20fdbd2@gmail.com>

openstack/neutron,openstack/neutron,mahak/neutron,mahak/neutron,mahak/neutron,openstack/neutron