Add possibility to monitor client nodes

[coder-hugo]

This PR adds a new command line flag -mounts-only which disables all checks except gluster_mount_successful and gluster_volume_writeable. This is useful for the monitoring of nodes that have just the client package of glusterfs installed to mount gluster volumes.
Furthermore I adapted the documentation for the boolean command line flags. The value true/false has to be set as follows for such flags: -flag-name=true/false (or can be left for true). I'm not sure whether this is a bug or a feature in go. You can find the relevant code for this here.

[ofesseler]

Thanks for participating!! I'll have a look in a few days.

[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.

[coder-hugo]

Yes I'd like to continue to develop this exporter. We are already using an enhanced version of this exporter in production and I think my adaptions would be also useful for others. So it would be great if we find a way to get my changes back to this repo.

[ofesseler]

I invited you as a collaborator. are you able to commit / accept pull requests?

[coder-hugo]

Yes, I am able to do this.

[ofesseler]

@coder-hugo Why didn't you accept your pull request? Are there any issues? May I help?
regards, Oli

[coder-hugo]

@ofesseler I just forgot about doing it. When I did my last answer I had not the time to check how to "become" a reviewer and approve the pull request. Without an approval I can't merge it.
Edit: I can't become a reviewer of my own pull request so I'll do the merge with the cli.
Edit2: Even with the cli I can't merge it without an approval. I'll have a deeper look into it later.