Support notifications after execution of commands

[jrbalderrama]

Add notifications support to any team collaboration tools (slack, mattermost) after the execution of any cli command. It may be implemented by adding an option enable_notifications = True|False in the configuration file.

The notification may include exit_status, executed command, OAR_JOBID among others

[msimonin]

Note that, without doing anything, this can be achieved by a ansible hook but will only notified for Ansible Task.