Rewriting Riemann output plugin (#1900)

* rename to riemann_legacy

Signed-off-by: Fabio Berchtold <fabio.berchtold@swisscom.com>

* initial draft for Riemann output plugin rewrite

Signed-off-by: Fabio Berchtold <fabio.berchtold@swisscom.com>

* add unit tests

Signed-off-by: Fabio Berchtold <fabio.berchtold@swisscom.com>

* add option to send string metrics as states

Signed-off-by: Fabio Berchtold <fabio.berchtold@swisscom.com>

* add integration tests

Signed-off-by: Fabio Berchtold <fabio.berchtold@swisscom.com>

* add plugin README.md

Signed-off-by: Fabio Berchtold <fabio.berchtold@swisscom.com>

* bump riemann library

* clarify settings description

Signed-off-by: Fabio Berchtold <fabio.berchtold@swisscom.com>

* update Readme.md with updated description

Signed-off-by: Fabio Berchtold <fabio.berchtold@swisscom.com>

* add Riemann event examples

Signed-off-by: Fabio Berchtold <fabio.berchtold@swisscom.com>

* use full URL for Riemann server address

Signed-off-by: Fabio Berchtold <fabio.berchtold@swisscom.com>

closes #1878