Fix conditionals for Log file union task
Fix conditionals for Log file union task

The task to perform a union on the log files will fail if the previous
task is skipped, since the value "log_files.files" is not defined.

We should skip the if the variable doesn't exist, since we already
conditionally perform the task that creates the "log_files" fact.

Change-Id: Iade36c8d79b0e0653c4f7348e609aa1f8d88a4a0

openstack/openstack-ansible-rsyslog_client,openstack/openstack-ansible-rsyslog_client