Make network/api.py use Network object for associations

This makes the network/api.py module use the Network object for
network host and project associate/disassociate operations. There
was very little testing of this, so this patch adds/corrects tests
to verify object behavior.

Related to blueprint compute-manager-objects-juno

Change-Id: I5c0d61be35fc4edf647e572c746515988651bcc0
Merge "Make network/api.py use Network object for associations"