Executes a command as a given user.
Command is executed using ssh if the user is not on localhost.
Before executing the command, the privileges will be elevated.
This privilege elevation method is defined by $RIGHT_ELEVATION in the
service configuration and can be either
base-exec-superuser USER COMMAND
USER: A key of an user definition in the repository. From this, the following properties are used:
RIGHT_ELEVATION: Method to evelate the privileges. Can be
COMMAND: A command to be executed. It is executed either locally or remote using base-ssh-login.
Returns the output of the command.