upgrades for ES 2

[sherzberg]

These changes allow gathering metrics from elasticsearch 2 instances. Prior to these changes, many warnings about missing stats would be logged.

[faxm0dem]

I didn't test this yet, but thanks!

[rgevaert]

This works and is backwards compatible. Only issue that I saw is that you now need to use X.Y.Z version numbers. In the original plugin it was enough to use X.Y version numbers. Now the plugin breaks. But it can be changed in the config file.

Also #17 is worth noticing. People running 2.X.Y are probably using the python elasticsearch module provided by Elastico and that clashes with the module naming...

[jnovack]

This works, and is backwards compatible, HOWEVER, not a drop-in replacement as the config needs to be modified for X.Y.Z as noted above.

<Plugin "python">
    ModulePath "/usr/lib64/collectd/"

    Import "elasticsearch"

    <Module "elasticsearch">
        Verbose false
-        Version "1.0"
+        Version "2.2.0"
        Cluster "elasticsearch"
    </Module>
</Plugin>

LGTM!!