This change passes the agent object into extensions, such that
they may call agent methods as needed. It also causes async
commands to force a heartbeat immediately after completing the
command. This allows Ironic to get a heartbeat and continue
work as quickly as possible, while also allowing deployers to
configure Ironic (agent) to heartbeat less often.
Change-Id: Ib3c3a43dfd0ed4ed51b7d52ac099f01181ca822f