(#21520) Provide option for soft failure during terminus writes when PuppetDB is unavailable

This patch adds a new option 'soft_write_failure' for puppetdb.conf to change
the terminus behaviour so that if a command or write fails, instead of
throwing an exception and causing the agent to stop it will simply log an
error to the puppet master log.

Signed-off-by: Ken Barber <ken@bob.sh>