Do not cache auth token in Neutron DHCP provider

Currently token expire after some time and deploy breaks.
Also refactored all tests for Neutron DHCP provider, as
many of them were just broken (e.g. using non-existing asserts).

Change-Id: I38a8e95a09fd4737682e37b1162eb445fbc8012f
Closes-Bug: #1371503
Merge "Do not cache auth token in Neutron DHCP provider"