Ticket #20: Add threshold support to manage warning status

* A new configuration parameter is introduced : 'warnings'. It allows to
configure the warning status threshold in addition of the errors property.
If nb_errors > errors : Action status is ERROR
If warnings < nb_errors < errors : Action status is WARNING
If nb_errors < warnings : Action status is OK
If warnings >= errors, warnings is ignored

* Manage the WARNING return code in the CLI.
* Manage the WARNING status for actions.

This patch fixes #20

Change-Id: I1b8f312674534203c6e0851bef32fe449dc1736c