Pool parsing fails with ceph 0.87

[benh57]

From the release notes:

The rd_kb and wr_kb fields in the JSON dumps for pool stats (accessed via the ceph df detail -f json-pretty and related commands) have been replaced with corresponding *_bytes fields. Similarly, the total_space, total_used, and total_avail fields are replaced with total_bytes, total_used_bytes, and total_avail_bytes fields.

This breaks the plugin

[benh57]

I'l probably just hack mine to look for the 0.87 versions but perhaps supporting both would be good.