removed unused method and added another test
removed unused method and added another test

get_network_id_by_cidr is not used, I'm removing
it and it's tests.

get_tenant_id_by_net_id relies on catching 
exceptions as part of the happy path. I expanded
the tests to try to exemplify that.

Change-Id: Iaf973da1d176ba8ea89d0fd762e796936d056d0f
n0ano/ganttclient