Create %{_datarootdir}/opentsdb/plugins in rpm.

The folder %{_datarootdir}/opentsdb/plugins is required for opentsdb to
start and should be created by the rpm when installing.

Signed-off-by: Chris Larsen <clarsen@euphoriaaudio.com>
Fix CLIQuery usage command for downsampling per #314

Signed-off-by: Chris Larsen <clarsen@euphoriaaudio.com>