[MONIT-4106] values/metrics/fluentbit: add default luaScripts value

In commit c6c3db28 we added support for lua scripts provided via user
values.

This commit adds a default value and documents this in the values file.

Reported-at: https://its.cern.ch/jira/browse/MONIT-4106


Signed-off-by: Guillermo Facundo Colunga <guillermo.facundo.colunga@gmail.com>
Signed-off-by: Guillermo Facundo Colunga <guillermo.facundo.colunga@cern.ch>