Remove Heartbeat-Before check from heartbeat

Ironic does not send this header - agent should not look for it.

Change-Id: I7b8857fa5f744bc03ee0907424a95b9305811c51
Fix unexpected stevedore traceback in tests

Fix tests/agent.py: make use of test stevedore manager to avoid loading of real extensions

Change-Id: I258e988e505d933b8359b62c0b65ebdb76dca8e5
Merge "Fix unexpected stevedore traceback in tests"
Merge "Remove Heartbeat-Before check from heartbeat"