qa/workunit: optionally let the DaemonWatchdog stop a Workunit

Before this commit, workunit did not implement WatchedProcess.
If a daemon fails and watchdog barks, the workunit would still keep
running against a broken cluster. This lead to long running jobs,
which ideally should have been terminated as soon as the watchdog
barked.

This commit registers Workunit as a WatchedProcess. This is beneficial
for the following reasons:

1. Watchdog can kill the registered workunit once it barks, thus
failing fast incase of a failure.
2. The actual reason for the failure can be surfaced from within the
workunit  instead of an ambiguous failure reason.

By default, all workunits have wactchdog disabled. This can be enabled
by setting watchdog:true (and for cephadm: watchdog_setup:true).

Signed-off-by: Shraddha Agrawal <shraddha.agrawal000@gmail.com>