Indicate a warning in the 'icinga' check when cluster stage validation failed

Indicate a warning in the 'icinga' check when cluster stage validation failed

- success: clear the last failed attribute
- failed: populate it with the output and current timestamp

This can be used to highlight this in the 'icinga' check task.
Since 2.9 we don't have problems with circular library dependencies
with just one linked binary, therefore it is safe to include libremote
in libmethods here.