initialize objects with context in network manager tests

These changes aim to clean up the pattern of passing a context in
object member functions create/destroy/refresh/save and instead
initialize the object with the context when it's constructed.

Related to blueprint kilo-objects

Change-Id: Ia27e0952d396f1a4c9c520faeeb18517a394ab9b
Merge "initialize objects with context in network manager tests"