Simplification orders of Bridge creation for OVS agent

Closes-Bug:  #1259924
* use neutron::agents::utils::bridges{} abstraction for create bridges for ovs-agent
* naming Cs_resource for more convinient use.
* remove unneeded relationship

Change-Id: I35540ebb27121111e00668862947e4ee6ab1ee99
Merge "Simplification orders of Bridge creation for OVS agent"