Remove and block DB access in dhcpbridge

This fixes an issue of direct DB access from happening in dhcpbridge
by using an object method instead. For this to work and to prohibit
future DB access from happening, the objects indirection API is set
and DB access is blocked in a similar way to how it is done in
nova-compute and nova-network.

Change-Id: I5e3152aad13343a6a1ecd9b5b511429159c9d4c2
Closes-Bug: #1316074
Merge "Remove and block DB access in dhcpbridge"