KeyError from AgentVendorInterface._heartbeat()

A KeyError exception could be raised in AgentVendorInterface._heartbeat()
if kwargs['agent_url'] doesn't exist. It *should* always exist, but just
in case, reraising a MissingParameterValue exception with a more specific
msg would be helpful in debugging.

Change-Id: I5e7971e7e8062d7917cc34eb9e0178778ed5035a
Closes-Bug: #1378440
Merge "KeyError from AgentVendorInterface._heartbeat()"