Add Atoi error handling to fix gosec warning.
Add Atoi error handling to fix gosec warning.

See,

$ make gometalinter
>> linting code
gluster_client.go:151::warning: Errors unhandled.,LOW,HIGH (gosec)
gluster_client.go:151::warning: Errors unhandled.,LOW,HIGH (gosec)
gluster_client.go:151::warning: Errors unhandled.,LOW,HIGH (gosec)
make: *** [Makefile:59: gometalinter] Error 1

ofesseler/gluster_exporter,ofesseler/gluster_exporter