Some tests for dhcp agent scheduler

Add some test cases for dhcp agent scheduler.

Change-Id: Ied0b0baa7a6005028dd0f51218a0996e12c9801e
Add internal testing for the stress test framework

This adds a unit test for StressAction class and a end2end test
case for the whole stress test framework.

Change-Id: I4d30a131c75736fa05ba63f71cd731374f7fb9ba
Merge "Some tests for dhcp agent scheduler"
Merge "Add internal testing for the stress test framework"