implement testDatasource function

[sspieker]

This PR adds a testDatasource() function, following the documentation for datasource plugins at Datasource development. It queries the health endpoint of KairosDB and returns the http status code.

This fixes 2 situations:

1. the misleading error message when clicking on "save & test" in the datasource configuration
2. the new Grafana 6 feature which does not allow a KairosDB datasource to be selected if the health check fails

[sspieker]

@jifwin @marefr , sorry to bother you but you're the persons who did the last commits in this repository. Could you please take a look at this? With this quite simple PR KairosDB becomes functional in Grafana 6 again.

[djzort]

you might consider also pushing to https://github.com/kairosdb/kairosdb-datasource