Add takeover delay to quantum-ha-tool

Add a takeover delay to the quantum-ha-tool to ensure we do
not try to aggressively takeover.  We want to ensure an agent
is offline for some period of time (TAKEOVER_DELAY seconds)
before we begin a migration.  We also randomize it a bit in the
event that this script is being run in parallel from several hosts.

Change-Id: Ied9a75791b27af310f81f4cec327b3699fbae7ed
Parameterize agent report timers

* Ensure that report_interval and agent_down_time
are both parameters that can be overriden
* Relax the default agent_down_time to 15 seconds
since 4 seconds (1 second > report_interval) proves
unreliable with a large number of routers

Change-Id: Id9bfcd01ba36553b4fb1371238d58de77435a9aa