Performance tests for handling nodes in Fuel Client

For performance testing all invotations to Nailgun
were mocked in order to only measure pure client's
performance.

The patch introduces the following tests for 100 nodes:

- list all nodes;
- assign nodes to an environment;
- list environment with pending nodes;
- upload network parameters to nodes.

Blueprint: 100-nodes-support
Change-Id: I849ecd17ba4a9a48b8d4e6dafa75b095ee919ce6
Merge "Performance tests for handling nodes in Fuel Client"