Redirect stderr > dev/null when looking for master

Prevent extraneous output from going to corosync log file.

When I have one out of three nodes in standby, the corosync log gets filled with stderr logs like so:

Feb 01 10:10:13 domain1.com lrmd: [6770]: info: RA output: (mysql_prm:0:monitor:stderr) resource ms_mysql is NOT running
Feb 01 10:10:18 domain1.com lrmd: [6770]: info: RA output: (mysql_prm:0:monitor:stderr) resource ms_mysql is NOT running

This if from the output of `crm resource list ms_mysql`

```
resource ms_mysql is running on: domain1.com Master
resource ms_mysql is NOT running
resource ms_mysql is running on: domain3.com
```

The `NOT running` line is stderr output and is being caught and logged to corosync's log file.

Redirect this command's stderr to dev/null to prevent this output.