A callback plugin that logs failure message

A callback plugin that logs failure message

This callback plugin is called on every failure and prints out the
failure information as yaml so that it's easier to read in the
teuthology logs.

If the environment variable ANSIBLE_FAILURE_LOG is present then a log of
all failures in the playbook will be persisted to the file
path given in the ANSIBLE_FAILURE_LOG.

Signed-off-by: Andrew Schoen <aschoen@redhat.com>