Convert network/api.py get calls to use Network object

This makes the network api module use the Network objects for
get() and get_all() operations. There were no tests for these methods,
so this patch adds them in the process.

Related to blueprint compute-manager-objects-juno

Change-Id: Iea1104a111e50a796077cdf0ed5b0f4e75bfcbcb
Merge "Convert network/api.py get calls to use Network object"