replace $ECHO variable with echo command

Hi

Fixes problem, that output message wants to be executed. E.g.:
`/usr/lib/nagios/plugins/check_glusterfs: line 64: CRITICAL: no bricks found: command not found`

I don't know where this `$ECHO` variable comes from. It's not defined within the script and also not in `utils.sh` from monitoring-plugins-2.0.

Cheers,
ganto