MCOP-310 Revert changes to the agent

This is a partial reversion of f8a7ca03 and 3033b48

Those commits were, rather than allowing a default to be supplied to the
puppet_agent_mgr, forcibly setting the signal_daemon option to be false, which
has far more reach than intended by the original proposed change.
MCOP-310 move the lookup of the default

The lookup of the default should happen at the last reasonable moment, so over
in Puppet_agent_mgr#runonce.  Here we also add testing so we can see the
difference in behaviour for both supported values.
MCOP-310 Document the default behavior of puppet.signal_dameon

Here we document the current default behavior of the signal_daemon
option, which matches the previous default unconfigurable value.

We tone down some of the polemic about the historic default, it's
not recommended given puppet 3.x's behaviour now allows a foreground
run alongside a daemon, but it's not going to eat any babies.
Merge pull request #40 from richardc/MCOP-310-signal_daemon_original_default

MCOP-310 restore signal_daemon's original default