Log: pacemaker-remoted: use different default log if pid 1

When pacemaker-remoted runs as pid 1 inside a container, there may not be a
/var/log/pacemaker directory. To get around this, use a default log of
/var/log/pcmk-init.log when running as pid 1.

This was chosen over alternatives (creating the /var/log/pacemaker directory,
or passing the log location as an environment variable when creating the
implicit container resource) because it both avoids forcing a restart of
active bundles due to configuration change (as well as preserving regression
test output) and allows users to configure an explicit log location via the
container image or the bundle's extra arguments.