Add no-heal scrape option

[insoIite]

Add option in order to not perform a heal info  …

When there is no healing files, the gluster volume heal info command is quite fast. But in case of problems it can take more than 10s to answer for only one volume. In this case your gluster exporter will not work (command answer time > scrape time etc...). Furthermore the load of your gluster server will rise very high.

[ofesseler]

Thanks for this pull request, unfortunately I have no time to test it. If you like to continue to develop this exporter please let me know.

[ofesseler]

good point! needs to be updated to current master.