Improved share_servers db api

Returning backend_details in dict.
Removed unused db method
share_server_get_by_host_and_share_net.
Added unittests.

Partially-implements bp implement-backend-details-in-drivers

Change-Id: I1998ce06e1568ea6318fad9c535832295bbddcaa
Merge "Improved share_servers db api"